The United States on Friday began building a circumstantial case against Russia for the downing of a Malaysian airliner, as President Obama said Russian supplies of sophisticated weapons to Ukrainian separatists were “not an accident.”

Confirming widespread reports of preliminary U.S. conclusions, Obama said the plane “was shot down” Thursday by a surface-to-air missile fired from separatist territory. All 298 aboard, including one U.S. citizen, were killed.

Obama stopped short of publicly accusing the separatists, or their Russian patrons, of pulling the trigger. But he left little doubt whom he believed was to blame for what he called “an outrage of unspeakable proportions.”

As an international inquiry was organized, and investigators struggled to reach the wreckage and bodies strewn across fields of wheat and sunflowers in separatist-held territory, Obama said Russian President Vladi­mir Putin has the power to end the escalating violence in Ukraine.

“If Mr. Putin makes a decision that we are not going to allow heavy armaments and the flow of fighters . . . across the Ukrainian-Russian border, then it will stop,” he said in remarks at the White House. “He has the most control over that situation, and so far, at least, he has not exercised it.”

Senior aides added elements to the administration’s case throughout the day. At an emergency U.N. Security Council meeting in New York, U.S. Ambassador Samantha Power said a Russian-made SA-11 missile system, easily capable of reaching the plane that was flying at 33,000 feet, had been spotted in the area of the shoot-down. Because of the system’s “technical complexity,” she said, “it is impossible to rule out Russian technical assistance” to separatists operating it.

Separatist leaders, who have downed several Ukrainian military aircraft in recent days, had “boasted” on social media about Thursday’s shoot-down “but later deleted those messages,” Power said. “Russia can end this war. Russia must end this war,” she said.

“Whether it was a Russian military unit that did it or it was a separatist unit . . . we don’t know,” Pentagon press secretary Rear Adm. John Kirby said at a briefing. He noted that Russia has up to 12,000 troops deployed on its side of the border with Ukraine.

Privately, U.S. officials said intelligence assessments, based on weapons believed to be in separatist hands and the tracked location of the launch site, had concluded that separatists fired the missile, although it was unclear whether they knew their target was a commercial airliner.

One of the strongest public statements came from Britain, which lost 10 citizens aboard the flight. A statement released by Prime Minister David Cameron’s office said “it is increasingly likely that [the Malaysian airliner] was shot down by a separatist missile.”

Russia said it welcomed an investigation by the United Nations’ International Civil Aviation Organization but responded sharply to the thinly veiled U.S. accusations of at least indirect responsibility for the shoot-down.

Obama should “stop lecturing Russia” and force the Western-backed Ukrainian government to seriously engage in negotiations with the separatists, Foreign Minister Sergei Lavrov said.

Aviation history is littered with civilian planes that were shot from the sky, intentionally or not, by military weapons. Malaysia Flight 17 was cruising at 33,000 feet, more than half a mile higher than Mt. Everest, when a missile hit it July 17. And the missile’s range is believed to be more than twice that high.

At the U.N., Russian Ambassador Vitaly Churkin criticized those he said were “trying to prejudge the outcome [of an investigation] with broad statements and insinuations” and accused the Ukrainian government of failing to warn international aviation to avoid the conflict area.

By continuing its military offensive to dislodge the separatists, Ukraine “chose the wrong path, and their Western colleagues supported them,” Churkin said. “I’m talking about the United States; they actually pushed them to escalate,” he said, and now “they are trying to lay the blame on Russia.”

A spokesman for the Russian Defense Ministry said none of its sophisticated antiaircraft systems, or any other weaponry in service with the Russian armed forces, had crossed the border into Ukraine, Itar-Tass reported from Moscow.

Amid the official statements and allegations came the now-familiar accoutrements of international tragedy — the laying of flowers at embassies and in hometowns, candlelight vigils and the signing of condolence books.

The one U.S. citizen known to be aboard the flight, Dutch dual-national Quinn Lucas Schansman, 19, was traveling on the Malaysian flight from Amsterdam to Kuala Lumpur to meet his family for a vacation.

Three Australian siblings, ages 12, 10 and 8, returning home with their grandfather from a vacation were among the 80 children the United Nations said were aboard the flight. Power, in her Security Council speech, said three children had the letter “I” next to their names on the flight manifest, indicating they were infants.

Two-thirds of the passengers were from the Netherlands, where grieving relatives gathered at Amsterdam’s Schiphol Airport. A large but undetermined number of the dead were international AIDS researchers traveling to a conference in Australia.

Referring to the researchers, Obama said that “in this world today, we shouldn’t forget that in the midst of conflict and killing, there are people like these — people who are focused on what can be built rather than what can be destroyed. People who are focused on how they can help people that they’ve never met. People who define themselves not by what makes them different from other people but by the humanity that we hold in common . . . and it’s time for us to heed their example.”

Although statements by the United States, and others such as Britain, grew harsher as the day progressed Friday, it was unclear what the administration proposed to do once responsibility for the attack was firmly established, and who would be with it.

“This should snap everybody’s heads to attention and make sure that we don’t have time for propaganda, we don’t have time for games. We need to know exactly what happened, and everybody needs to make sure that we’re holding accountable those who — who committed this outrage,” Obama said. He said he saw no U.S. military role in the conflict.

Several governments represented at the Security Council meeting limited themselves to expressing shock and sympathy over the dead. China cautioned member nations not to “jump to any conclusions . . . or trade accusations.”

German Chancellor Angela Merkel on Friday pressed Russia to work harder toward a political solution in Ukraine. But she also drew a line between the Russians and the separatists, saying that “the Russian president of course has an influence,” but “still one has to differentiate between the separatists and the Russian government.”

On Wednesday, both the United States and the European Union imposed new sanctions on Russia, but the European moves were significantly less stinging. Suggesting Friday that she was in no rush to go further, Merkel called Wednesday’s move “an adequate response to what happened in the past few days,” although she noted that the European decision had left open the door to “act on a new level” if necessary.

European governments have generally been more reluctant than the Americans to slap tougher sanctions on the Russians largely because of Moscow’s economic clout in the region. Some analysts suggested that the desire of Europeans to sidestep truly forceful sanctions to protect their economies should not be underestimated.

But should the separatists, with direct or indirect Russian aid, be conclusively proved responsible for the shoot-down, others said the calculations in the region might change.

“It’s a game-changer because it’s very difficult to see how anyone in Europe can continue business as usual,” said Jonathan Eyal, director of international security studies at the Royal United Services Institute in London. “This time we have our own European casualties. It’s not a theoretical conflict in which people we do not know are dying.”

Anthony Faiola in Berlin contributed to this report.