Barack Obama threatened to "increase the costs" to Russia if Moscow fails to de-escalate the situation in Ukraine, as US and other western officials said there was mounting evidence that a missile fired by Russia-backed separatists downed the Malaysia Airlines jet which crashed in eastern Ukraine on Thursday.

All sides have called for a thorough and impartial investigation into what caused the crash, which killed all 283 passengers and 15 crew on board MH17, en route from Amsterdam to Kuala Lumpur. However, the first group of international investigators on the scene were involved in a tense standoff with the armed separatists who control the crash site at Grabovo.

A unit of heavily armed rebels blocked the 30-strong team from the Organisation for Security and Co-operation in Europe (OSCE), cordoning off a large part of the crash site. The inspectors retreated after an hour-long standoff, having been unable to gain access. They were sent on their way by warning shots fired by the rebel unit commander.

"We will keep coming back tomorrow and the next day and the next day," said OSCE spokesman Michael Bociurkiw. "Tomorrow will be a crunch day. There are a lot of experts from the Netherlands and Malaysia gathering in Kiev as well as relatives. The bodies are starting to bloat and decay. An expert team is clearly needed. There is a lot to be done in a short amount of time."

In Washington, Obama called for a full, impartial investigation and said the tragedy should cause people to "snap their heads together" and stop playing games in Ukraine. In veiled criticism of the lack of European support for US-led economic sanctions against Russia, Obama said the loss of so many European lives should serve as a wake-up call for Europe.

He said while it was too early to be completely sure who was responsible and what their motives were, the US was certain that a missile fired from within territory controlled by Russian separatists brought down the jet. He stopped short of directly blaming Moscow for the tragedy, but said it was down to Vladimir Putin to end the violence that has plagued Ukraine for months.

"It is not possible for the separatists to function the way they're functioning … without sophisticated equipment and training, and that is coming from Russia," said Obama. "If Mr Putin makes a decision that we are not going to allow heavy armaments and the flow of fighters into Ukraine … then it will stop."

In the UK the government joined the US in blaming pro-Russian separatists. A No 10 spokeswoman said: "While it is too early to be categoric about the cause of the disaster, the growing weight of evidence suggests that MH17 was shot down by a surface to air missile and that this was fired from near Torez, in territory controlled by the separatists."

Russia, however, did not appear eager to disown the rebels, to whom it has been publicly sympathetic and in private supplied logistical and military backing – or at least turned a blind eye to their provision across the Russia-Ukraine border.

Russia's ambassador to the UN, while not addressing the specific claim of how MH17 was shot down, said Russia "fully blames Kiev" for all violence in the region. In Moscow, foreign minister Sergei Lavrov dismissed claims from Kiev that Russia was behind the attack, saying that he had "hardly heard a single true statement come from Kiev in the past few months".

Lavrov's deputy, Sergei Ryabkov accused the Americans of having made up their minds without studying the facts, and insisted that the US that was to blame. Washington, said Ryabkov, was responsible for "stirring up political instability, provoking an anti-constitutional seizure of power and supporting anti-Russian politicians … The US should think about the consequences of its actions."

Russia's state-controlled media suggested that a Ukrainian jet or missile system had shot down the plane, with a source in Russia's defence ministry claiming that Moscow had picked up missile radar activity on Thursday coming from Ukrainian bases. Other more fanciful conspiracy theories were floated, including the idea that the attack was carried out by the Ukrainian army in error, thinking it was Putin's plane. Rebel websites suggested that the bodies discovered at the crash site were "long dead" and speculated the plane could have been MH370, which went missing earlier this year, hidden and then re-used to stage a "provocation".

While Obama admitted that the US does not know exactly what happened, amid the ludicrous theories the circumstantial evidence did appear to point increasingly to an accidental attack by separatists who thought they were shooting at a Ukrainian military jet.

If the missile was fired by rebels, it is unclear if they obtained the launch system from Russia or seized it from a Ukrainian army base. Video posted on YouTube claiming to show part of a Buk system on the move towards the Russian border could not be verified. Ukraine's foreign minister, Pavlo Klimkin, however, insisted that recordings released by Ukraine's SBU security services on Thursday of separatist fighters admitting they had downed a passenger liner were "absolutely genuine".

Elsewhere in the east, the fighting between Kiev forces and separatists continued. There were reports that at least 20 civilians had been killed by shelling in the city of Luhansk.

The 298 people aboard MH17 came from nearly a dozen nations, with at least 189 of the dead Dutch citizens. A large number were heading to a conference on HIV/Aids.

Murdo Bijl, a Dutch Aids advocate who knew many of those on board, said: "Truly beautiful, inspiring, committed, smart and compassionate people have been brutally taken away from us. The world and the Aids field will miss these brilliant doctors, advocates, researchers and friends."

The Dutch prime minister, Mark Rutte, who on Thursday night had urged caution about jumping to conclusions, by Friday afternoon was calling for consequences. "Let us be clear: if it becomes clear that it was an attack, the perpetrators must be found and punished," he said at a press conference at the ministry of security and justice. "That is something the victims and those they leave behind are entitled to."

Yet, over all, the reaction to the tragedy in the Netherlands has been remarkably restrained. morning at 8am, passengers were queuing up to check into another Malaysia Airways flight bound for Kuala Lumpur. One couple in their 20s, who didn't want to give their names, said they felt sad about what had happened, and admitted they were scared about boarding their flight. But they were determined not to give up on their holiday, a trip around Indonesia and Sumatra.

Erik Elsenaar, an IT consultant waiting at Schiphol for his midday flight to Kuala Lumpur, said he was feeling very calm: "This is something that never, never happens, and it's unlikely to happen again. It is a tragedy, but it doesn't look like the attack was either directed at Holland or Malaysia Airlines. You can see here that they've already doubled the security at check-in. They will probably triple the checks for bombs. It's definitely safer to travel now than it was to travel the day before yesterday."

There were nine UK citizens among the dead. A British diplomat in Kiev said if any of the relatives of the nine UK citizens killed in the crash came to Ukraine, they would be given all assistance required, even as far as attempts to journey to the crash site, "within the limits of what is safe, possible and accessible".

A video-conference took place on Thursday evening between the separatists and the Ukrainian president's representative, former president Leonid Kuchma, which included mediation from the OSCE and the Russian ambassador to Ukraine. It agreed that the separatists would grant access to the crash site for international investigators. However, Friday's standoff with the OSCE shows that unfettered access could be tricky to achieve, and the logistics of issues such as retrieving and properly storing bodies, as well as sifting through the evidence, remain unclear.