(Adds details on Obama calls with European leaders)
By Jeff Mason and Steve Holland
WASHINGTON, July 18 (Reuters) - President Barack Obama said on Friday the downing of a Malaysian jetliner in a Ukrainian region controlled by Russian-backed separatists should be a "wake-up call" for the West in its drive to hold Moscow accountable for a crisis that appears to be at a turning point.
While stopping short of blaming Russia for Thursday's crash of Malaysia Airlines flight MH 17, in which 298 people died, Obama accused Russia of failing to stop the violence that made it possible to shoot down the plane. The United States has said the jetliner was hit by a surface-to-air missile fired from rebel territory.
A senior U.S. official said there was increasing confidence that the missile was fired by separatists and that there was no reason to doubt the validity of a widely circulated audiotape in which voices identified as separatists discussed the downing of the plane.
"This certainly will be a wake-up call for Europe and the world that there are consequences to an escalating conflict in eastern Ukraine; that it is not going to be localized, it is not going to be contained," Obama told reporters.
Obama spoke by phone later with German Chancellor Angela Merkel, British Prime Minister David Cameron and Australian Prime Minister Tony Abbott. The White House said they discussed Ukraine and the downed jet and the need for an unimpeded international investigation into what happened.
Increasing international demands for an investigation into the crash present Obama with the opportunity, at least temporarily, to counter the perception that his presidency's global influence is eroding and to exert leadership at a time when his domestic and foreign policy initiatives appear to be faltering.
Television broadcasts of pro-Russian rebels sifting through the remains of the Boeing 777-200 divert attention from a crisis in the United States involving child migrants on the southern U.S. border and crises abroad including Israel's ground assault in Gaza and Islamist gains in Iraq.
Under siege by critics who pan his global approach as ineffective, Obama faces a test of whether he can whip up enough international support to help defuse one of the biggest crises of his presidency: the worst collapse in East-West relations since the end of the Cold War.
Obama toughened his message to Russian President Vladimir Putin a day after Putin complained in a phone call to Obama about new U.S. sanctions directed in part at Russia's energy sector.
In an example of how Obama and Putin are still able to communicate with each other despite months of bitter differences, it was revealed on Thursday that Obama learned of the airline disaster from Putin during their phone call.
Now, Obama's goal is to tighten the clamps on Russia and force Putin to back down, something the Russian president has refused to do since the Ukraine crisis erupted earlier this year.
Obama must also convince German Chancellor Angela Merkel and other European leaders to take a more aggressive posture against Russia's activities in Ukraine in the wake of the Malaysia Air crash.
European leaders, sensitive to the damage Russia could inflict on their economies by, for example, cutting off natural gas supplies, have imposed sanctions that are less ambitious than the United States would like.
Obama and Merkel agreed in their call on Friday to stay in touch as they considered what additional actions against Russia were required, the White House said. Meanwhile, Cameron and Obama agreed on the need for more sanctions if Russia continued to fail to de-escalate the situation in Ukraine.
In his remarks to reporters Obama called the crash a "global tragedy."
Putting pressure on Moscow, Obama told the news conference, "the violence that's taking place there is facilitated in large part because of Russian support. If Mr. Putin makes a decision that we are not going to allow heavy armaments and the flow of fighters into Ukraine across the Ukrainian-Russian border, then it will stop.
"He has the most control over that situation," Obama said of Putin. "And so far, at least, he has not exercised it."
It was a sharp contrast to Obama's "reset" of relations with Moscow in his first term, once seen as a big achievement but criticized by Republicans who called Obama naive for trusting Putin.
Obama's display of resolve on Friday was far sharper than his initial reaction on Thursday when, as videos of smoke from the crash and the debris field were shown on U.S. television, he said it looked like it might be a terrible tragedy.
U.S. Ambassador to the United Nations Samantha Power issued an indictment of Moscow at a U.N. Security Council meeting, saying Russia has in the last few weeks increased the number of tanks, armored vehicles and rocket launchers in southwest Russia. More advanced air defense systems have also arrived.
"This war can be ended. Russia can end this war. Russia must end this war," Power said.
Foreign policy issues have regularly leapt to the top of Obama's agenda in spite of his attempts to focus on U.S. domestic policy in the run-up to congressional elections in November that he hopes will thwart Republican efforts to take over the U.S. Senate. Ukraine has topped the list for much of the year. (Additional reporting by Susan Heavey and Emily Stephenson; Editing by Jason Szep and Ken Wills)