Barack Obama has seized on the shooting down of the Malaysian passenger jet to redouble international pressure on Russia over its support for Ukrainian separatists, calling it a “wake-up call” for Europe.



The US president claimed the incident – coming less than 24 hours after the US announced a third wave of economic sanctions against Russia – demonstrated the risks of supplying heavy weapons and support to the rebels and how the impact was “not going to be localised”.



While the Pentagon said new military assistance to Ukraine would soon arrive, it continued to stop short of providing weaponry.



The US and its European allies have presented a united front against Russian aggression in recent months, but there have been significant divisions behind the scenes over the types of sanctions that should be levelled against Moscow.



“In just one country alone – our great allies the Dutch – more than 150 of their citizens have been killed and that sadly brings home the degree to which the stakes are high for Europe, not simply the Ukrainian people,” said Obama. “We have to be firm in our resolve in making sure that we are supporting Ukraine's efforts to bring about a just ceasefire.”



Although stressing further investigation was necessary to identify individuals involved, the president said it was a “sombre and appropriate time for us all to step back and take a look at what has happened” and dismissed Russian efforts to blame the Ukrainian government.



“This should snap everybody's heads to attention and make sure we don't have time for propaganda,” he said. “We don't have time for games.”



Asked if the tragedy would harden Europe’s position on sanctions against Moscow, Jen Psaki, the state department’s chief spokesperson, replied: “We can’t make a full prediction of that, for obvious reasons.”



But she added: “Certainly, seeing the horrific events that happened yesterday, seeing the families who are mourning their loved ones – all of the information that is available should be a wake-up call for everybody … We’ve been engaged in discussions with the Europeans about sanctions for months now. We’ll see how this proceeds.”



The most detailed version of the American case against Russia was presented by the US envoy to the United Nations, Samantha Power, at a highly charged security council meeting in New York.



Power pointed to initial boasts by separatists that they had shot down a Ukrainian military plane, which were removed from social media sites after it became apparent that a civilian airliner had been hit. She also said an SA-11 anti-aircraft system capable of shooting down a plane at MH17 altitude had been spotted a few hours before the disaster in a rebel-held area close to the crash site.



Although the Ukrainian military possesses such weapons, none of its missiles were in the vicinity and it had not fired a single anti-aircraft missile since the start of the conflict. In the most pointed US accusation towards the Russians to date, she added: “Because of the technical complexity of the SA-11, it is unlikely that the separatists could effectively operate the system without assistance from knowledgeable personnel. Thus we cannot rule out technical assistance from Russian personnel in operating the systems.” The Pentagon later said the notion separatists used a sophisticated surface-to-air weapon without technical assistance from Moscow “strains credulity”.



Power's Russian counterpart, Vitaly Churkin, responded by saying Russia “places all blame” on the Ukrainian government. However, he did not directly deny that Russian-backed separatists may have fired the missile.



He argued instead that Kiev had created conditions for the tragedy by its counterinsurgency in eastern Ukraine, and questioned why Ukrainian air traffic controllers had allowed the plane to fly over a war zone.



In Washington, Obama stressed that while it was too early to reach concrete conclusions, “there are only certain types of anti-aircraft missiles that can reach up 30,000 feet and shoot down a passenger jet”.



“It is not possible for these separatists to function the way they are and have the equipment they have [without Russian support],” he added. “Set aside what has happened with respect to the Malaysian airlines [plane] a group of separatists cannot shoot down military transport planes … without sophisticated equipment and sophisticated training and that is coming from Russia.”



Despite early reports that there may have been 23 US citizens abroad, Obama announced only one – Quinn Lucas Schansman, a Dutchman with dual, US citizenship – had been confirmed. The president argued the shooting of the aircraft was a “global tragedy” that required a “credible international investigation”.



Obama said while it was too early to be completely sure who was responsible and what their motives were, the US was sure that a missile fired from within territory controlled by Russian separatists brought down the jet.



“What we have confidence in saying right now was that a surface-to-air missile was fired and that's what brought the jet down,” he said. “We know that that shot was taken within territory controlled by the Russian separatists but it's very important we don't get out ahead of the facts and at this point.”



But Obama added: “Everyone needs to make sure we are holding accountable those responsible for this outrage.”



Still, the US administration was wary of involving itself directly in the conflict. Obama said there would be no "military role beyond what we've already been doing."



That includes providing some $33m worth of radios, body armour, first aid kits and other non-lethal items to the Ukrainian military, something the Pentagon said would continue. Soon to follow would be night-vision goggles and explosive ordnance disposal robots, said Rear Adm. John Kirby, the Pentagon press secretary.



Kirby said the Pentagon was continuing to review Ukraine's requests for military aid, which includes guns and other weapons to attempt to equalise the balance of force against the Russian proxies on its territory. But, Kirby said, "the focus of our assistance to date has been and remains on the non-lethal".



Russia has up to 12,000 troops on its side of the Ukrainian border, Kirby said, who were actively aiding the separatists. He stopped short of saying he had indications that the Russian battalions had moved Buk missiles across the border, something the Nato commander, Air Force General Philip Breedlove, told reporters on 30 June that he expected.



Speaking earlier in Berlin, the German chancellor, Angela Merkel, said the tragedy underscored, once again, that Russia should be held responsible for the instability in Ukraine, but she responded cautiously to suggestions that Europe should follow Washington’s lead by expanding sanctions.



“Regarding sanctions, I'd like to point out that the events with the plane, as far as I remember, were not even 24 hours ago and at the moment we need to sort out an independent investigation,” she told reporters.

"So it's perhaps premature to draw conclusions before we have access to the remains of the plane." EU ministers will meet on Tuesday to discuss a possible package of new punitive measures against Russia, amid continued wariness in European capitals about the economic consequences of more stringent sanctions.