The downing of a Malaysian commercial airliner flying at 33,000 feet over Ukraine could dramatically broaden the Ukrainian crisis, even before it is determined who bears responsibility.

What has been a months-long shooting war between the U.S.-backed government in Kiev and Russian-supported separatists — and a war of words and sanctions between the West and Russia — now includes the deaths of nearly 300 people from several nations.

Britain, which a Malaysia Airlines manifest indicated had nine citizens aboard the aircraft, has called for an emergency meeting Friday of the U.N. Security Council. Although no Americans were initially reported aboard, early information from the manifest accounted for only 242 of 283 passengers aboard. Fifteen crew members also were aboard.

In the Netherlands, where Malaysia Airlines Flight 17 took off from Amsterdam on Thursday en route to Kuala Lumpur carrying more than 154 Dutch citizens, Prime Minister Mark Rutte rushed home from a vacation.

“I am deeply shocked,” Rutte said in a statement. “Very much is still unclear about the facts, the circumstances and the passengers.”

Other fatalities included citizens from across a wide swath of Europe, East Asia and Australia.

“This is a new element that nobody expected,” James F. Collins, a former U.S. ambassador to Russia who now works at the Carnegie Endowment for International Peace, said of the plane’s downing. “It’s one of those events . . . that can have unpredicted negative or positive consequences.”

On the negative side, it marks a clear escalation of both firepower and the willingness to use it that could draw the patrons of both sides into more overt participation on the ground and more direct confrontation with each other.

Damon Wilson, an expert on Ukraine at the Atlantic Council, said that evidence tying the missile to the separatists, and by extension their Russian backers, would probably fundamentally alter the international community’s view of the conflict.

World leaders, including some U.S. allies in Europe, who have seen the conflict as a regional one and been reluctant to turn on Moscow could be forced to reassess their position, said Wilson, who worked on European policy at the White House between 2007 and 2009. “It’s pretty difficult to continue playing that game if you have clear Russian fingerprints on the shooting down of a civilian airliner,” he said.

Former secretary of state Hillary Rodham Clinton said in an interview with Charlie Rose that “if there is clear evidence linking Russia . . . that should inspire the Europeans to do much more” to punish Russia and assist the Ukrainian government.

But Collins and others suggested that the shocking nature of the incident could also be a wake-up call to all involved. “It may bring certain people to decide that some different approach is needed because this is really getting out of hand,” Collins said. “All of a sudden, it could mean a lot more people talking about [the Ukraine situation] and saying enough is enough.”

Both the Ukrainian government and the separatists pointed the finger at each other, and Russian President Vladi­mir Putin indirectly accused Kiev, saying that if it weren’t fighting the separatists that have taken over much of the eastern part of the country, no one would be shooting.

The United States and its allies were hesitant to quickly assign blame, and there was no overt suggestion that a civilian aircraft had been intentionally targeted. But there was a clear undercurrent in the Western response that the separatists were believed to be responsible.

“While we do not yet have all the facts, we do know that this incident occurred in the context of a crisis in Ukraine that is fueled by Russian support for the separatists, including through arms, materiel, and training,” said a statement released by the White House Thursday night after an extended meeting of President Obama’s senior staff.

The West has charged Russia with sending increasingly sophisticated weapons into eastern Ukraine. As recently as Wednesday, when Obama announced stepped-up sanctions against Moscow, officials cited extensive surveillance showing new Russian arms shipments and additional Russian troops deployed to the border.

U.S. officials said Thursday that analysts at the CIA and other intelligence agencies were scrutinizing intercepted communications, satellite imagery and other information in an effort to determine who fired the missile that struck the Malaysian aircraft.

The officials noted that the conflict has been an area of intense focus for U.S. spy agencies, which have stepped up efforts to monitor communications among rebel units as well as aerial surveillance to track positions of Russian-backed forces and their weapons.

In the wake of the crash, officials said, U.S. analysts were working to retrace the path of the missile and reviewing signals intercepts for telltale reactions or admissions among separatist units.

The crash site is in territory controlled by the separatists, and Putin offered assistance in investigating. The United States and its allies also offered assistance, and expressed concern that the site would be tampered with before facts are determined.

Ukrainian President Petro Poroshenko and Obama emphasized during a telephone call Thursday that “all evidence from the crash site must remain in place on the territory of Ukraine until international investigators are able to examine all aspects of the tragedy,” a White House statement said.

In a separate call to Malaysian Prime Minister Najib Razak of Malaysia, Obama expressed condolences and “underscored that the United States stands ready to provide any assistance or support necessary,” the White House said.

British Foreign Secretary Philip Hammond called for an international investigation and proposed that it be led by the United Nations’ civil aviation organization.

“We’re determined to get to the bottom of understanding what has happened here,” he said.

In its statement, the White House called for “a full, credible, and unimpeded international investigation as quickly as possible” and urged all parties, including Russia, to support an immediate cease-fire to facilitate the recovery of remains.

“In the meantime,” the statement said, “it is vital that no evidence be tampered with in any way and that all potential evidence and remains at the crash site are undisturbed. The United States remains prepared to contribute immediate assistance to any international investigation, including through resources provided by the NTSB [National Transportation Safety Board] and the FBI.”

Sen. Dianne Feinstein (D- Calif.) said after an intelligence briefing on Capitol Hill that “we hope to have more information within the next day or so.” But, she said, “if evidence emerges that Russia was involved, that would obviously be extremely concerning.”

House Intelligence Committee chairman Mike Rogers (R-Mich.) said that U.S. intelligence would have a better idea of who was responsible within the next 24 hours. “Right now, they would have no way of trying to reconstruct that without any forensic evidence,” Rogers said.

Collins, the former ambassador, emphasized that “nobody knows the facts yet, other than the obvious one that a plane went down with 300 people. It clearly didn’t just fall out of the sky.”

Greg Miller, Ernesto Londoño and Julie Tate contributed to this report.