Ukraine accused pro-Russian separatists of shooting down a Malaysian jetliner with 295 people aboard Thursday, sharply escalating the crisis and threatening to draw both East and West deeper into the conflict. The rebels denied downing the aircraft.

American intelligence authorities believe a surface-to-air missile brought the plane down but were still working on who fired the missile and whether it came from the Russian or Ukrainian side of the border, a U.S. official said.

A Troy University professor believes that regardless of who shot down Malaysian Airlines Flight 17 over the Ukraine on Thursday, the incident may prove to be a “game changer” in the region’s ongoing confrontation.

Michael O. Slobodchikoff, an expert on Russia and Ukraine, said there were three options if the plane was shot down.

“One is a Ukranian army accidentally shot it down; one is the Ukranian Separatists did it; or it was Russia,” he said. “I dont think they purposely shot down a passenger plane. Flying at 10,000 meters above, you can’t really tell what it is.”

Slobodchikoff, who specializes in relations between Russia and the former Soviet states, international conflict and peace and comparative politics, said the plane was most likely mistaken for a Russian military aircraft.

He does not think Russia shot the plane down.

“The Russian Air Force would have known that it was a regular passenger plane,” Slobodchikoff said. “And Russia didn’t really need to aggravate the situation any further than needed.”

Bodies, debris and burning wreckage of the Boeing 777 were strewn over a field near the rebel-held village of Hrabove in the Donetsk region of eastern Ukraine, about 40 kilometers (25 miles) from the Russian border, where fighting has raged for months.

The aircraft appeared to have broken up before impact, and there were large pieces of the plane that bore the red, white and blue markings of Malaysia Airlines — now familiar worldwide because of the still-missing jetliner from earlier this year.

The cockpit and one of the turbines lay at a distance of one 1 kilometer (more than a half-mile) from one another. Residents said the tail had landed around 10 kilometers (six miles) farther away. Rescue workers planted sticks with white flags in spots where they found human remains.

There was no indication there were any survivors from Flight 17, which took off shortly after noon Thursday from Amsterdam to Kuala Lumpur with 280 passengers and a crew of 15. Malaysia’s prime minister said there was no distress call before the plane went down and the International Civil Aviation Organization had declared that the flight route was safe.

President Petro Poroshenko called it an “act of terrorism” and demanded an international investigation. He insisted that his forces did not shoot down the plane.

Ukraine’s security services produced what they said were two intercepted telephone conversations that showed rebels were responsible. In the first call, the security services said, rebel commander Igor Bezler tells a Russian military intelligence officer that rebel forces shot down a plane. In the second, two rebel fighters — one of them at the crash scene — say the rocket attack was carried out by a unit of insurgents about 25 kilometers (15 miles) north of the site.

Neither recording could be independently verified.

Earlier in the week, the rebels had claimed responsibility for shooting down two Ukrainian military planes.

“While it’s too early for any definitive assessments, the incident could force the European Union to add more sanctions against Russia, prompting a Russian response and an escalation in the conflict,” said Slobodchikoff.

He would advise American officials not to act hastily. “Slow down and make sure that they know all the details before they rush into anything with Russia.”

President Barack Obama called the crash a “terrible tragedy” and spoke by phone with Russian President Vladimir Putin. Britain asked for an emergency meeting of the U.N. Security Council on Ukraine.

Later, Putin said Ukraine bore responsibility for the crash, but he didn’t address the question of who might have shot it down and didn’t accuse Ukraine of doing so.

“This tragedy would not have happened if there were peace on this land, if the military actions had not been renewed in southeast Ukraine,” Putin said, according to a Kremlin statement issued early Friday. “And, certainly, the state over whose territory this occurred bears responsibility for this awful tragedy.”

Officials said more than half of those aboard the plane were Dutch citizens, along with passengers from Australia, Malaysia, the United Kingdom, Germany, Belgium, the Philippines and Canada. The home countries of nearly 50 were not confirmed.

The different nationalities of the dead would bring Ukraine’s conflict to parts of the globe that were never touched by it before.

Ukraine’s crisis began after pro-Moscow President Viktor Yanukovych was driven from office in February by a protest movement among citizens wanting closer ties with the European Union.

“It’s a civil war going on right now,” said Slobodchikoff.

Russia later annexed the Crimean Peninsula in southern Ukraine, and pro-Russians in the country’s eastern regions began occupying government buildings and pressing for independence. Moscow denies Western charges it is supporting the separatists or sowing unrest.

“Separatists have argued they don’t have a weapon capable of reaching an airliner’s altitude,” said Slobodchikoff. “But if it’s proven they do, Russia could find itself in a precarious position for supplying advanced weaponry to the separatists.”

The RIA-Novosti agency on Thursday quoted rebel leader Alexander Borodai as saying discussions were underway with Ukrainian authorities on calling a short truce for humanitarian reasons. He said international organizations would be allowed into the conflict-plagued region.

Some journalists trying to reach the crash site were detained briefly by rebel militiamen, who were nervous and aggressive.

Aviation authorities in several countries, including the FAA in the United States, had issued warnings not to fly over parts of Ukraine prior to Thursday’s crash, but many airliners had continued to use the route because “it is a shorter route, which means less fuel and therefore less money,” said aviation expert Norman Shanks.

Within hours of Thursday’s crash, several airlines said they were avoiding parts of Ukrainian airspace.

Malaysia Airlines said Ukrainian aviation authorities told the company they had lost contact with Flight 17 at 1415 GMT (10 a.m. EDT) about 30 kilometers (20 miles) from Tamak waypoint, which is 50 kilometers (30 miles) from the Russia-Ukraine border.

A U.S. official said American intelligence authorities believe the plane was brought down by a surface-to-air missile but were still working to determine additional details about the crash, including who fired the missile and whether it came from the Russian or Ukraine side of the border.

But U.S. intelligence assessments suggest it is more likely pro-Russian separatists or the Russians rather than Ukrainian government forces shot down the plane, according to the official, who spoke on condition of anonymity because he was not authorized to discuss the matter publicly.

The U.S. has sophisticated technologies that can detect missile launches, including the identification of heat from the rocket engine.

Anton Gerashenko, an adviser to Ukraine’s interior minister, said on his Facebook page the plane was flying at about 10,000 meters (33,000 feet) when it was hit by a missile from a Buk launcher, which can fire up to an altitude of 22,000 meters (72,000 feet). He said only that his information was based on “intelligence.”

Igor Sutyagin, a research fellow in Russian studies at the Royal United Services Institute, said both Ukrainian and Russian forces have SA-17 missile systems — also known as Buk ground-to-air launcher systems.

Rebels had bragged recently about having acquired Buk systems.

Sutyagin said Russia had supplied separatists with military hardware but had seen no evidence “of the transfer of that type of system from Russia.” The weapons that the rebels are known to have do not have the capacity to reach beyond 4,500 meters. (14,750 feet)

A launcher similar to the Buk missile system was seen by AP journalists earlier Thursday near the eastern town of Snizhne, which is held by the rebels.

Poroshenko said his country’s armed forces didn’t shoot at any airborne targets.

“We do not exclude that this plane was shot down, and we stress that the Armed Forces of Ukraine did not take action against any airborne targets,” he said.

The Kremlin said Putin “informed the U.S. president of the report from air traffic controllers that the Malaysian plane had crashed on Ukrainian territory” without giving further details about their call. The White House confirmed the call.

Separatist leader Andrei Purgin told the AP he was certain that Ukrainian troops had shot the plane down, but gave no explanation or proof.

Purgin said he did not know whether rebel forces owned Buk missile launchers, but said even if they did, they had no fighters capable of operating them.

In Kuala Lumpur, several relatives of those aboard the jet came to the international airport.

A distraught Akmar Mohamad Noor, 67, said her older sister was coming to visit the family for the first time in five years. “She called me just before she boarded the plane and said ‘see you soon,’“ Akmar said.

It was the second time a Malaysia Airlines plane was lost in less than six months. Flight 370 disappeared in March en route from Kuala Lumpur to Beijing. It has not been found, but the search has been concentrated in the Indian Ocean far west of Australia.

There have been several disputes over planes being shot down over eastern Ukraine in recent days.

Associated Press contributed to this report.