Rescuers at the site of the Malaysia Airlines crash in Ukraine say they have recovered 251 bodies.
Almost 300 people lost their lives in the disaster on Thursday.
The site is controlled by pro-Russian rebels who are believed to be stopping investigators reaching parts of it.
Russia's president Vladimir Putin has been facing growing pressure from Western leaders, and now says he will help international observers get full access.
Putin’s closest allies could be targeted by European Union sanctions as the West grows increasingly frustrated with Russia’s leader.
British Prime Minister David Cameron, who is expected to make a Commons statement on the disaster, told the Russian president his support for the rebels in eastern Ukraine had “contributed to an appalling tragedy” and the delay in experts being able to carry out their investigation was “indefensible”.
Further pressure will be put on Moscow at the United Nations today and a meeting of EU foreign ministers tomorrow is expected to result in a more punitive sanctions regime, with Mr Putin’s “crony group” possible targets.
Mr Cameron, who has been trying to speak to Mr Putin since the Malaysia Airlines plane was brought down with the loss of 298 lives last week, finally had a 30-minute conversation with the Russian premier, an exchange which Downing Street insiders described as “frank”.
A No 10 spokeswoman said Mr Cameron had made clear to Mr Putin that the shooting down of MH17 was “totally unacceptable”.
Mr Cameron told the president that the “world was now watching” and he “must change course and work to bring stability to eastern Ukraine”, the spokeswoman said.
She said: “The evidence suggested that pro-Russian separatists were responsible and the Prime Minister made clear that if Russia wants to put the blame elsewhere they would need to present compelling and credible evidence.
“The PM made clear that our priority is to get experts to the crash site so they can recover and repatriate the victims and collect any evidence necessary for the investigation.
“The PM emphasised that the families of 298 individuals need to know that everything is being done to make this happen and called on president Putin to use his influence on the pro-Russian separatists to ensure this happens.
“The delay and restrictions so far were completely unacceptable and indefensible.
“The PM said that the tragedy had brought into sharp focus the consequences of destabilisation in eastern Ukraine.
“The G7 and EU had repeatedly called on president Putin to cease support for the separatists and to work with the rest of the world to find a peaceful resolution. Russia’s failure to do so had contributed to an appalling tragedy.”
The separatists placed bodies from the downed Boeing 777 in refrigerated train cars in the rebel-held town of Torez, nine miles from the crash site, and said they would remain there until the arrival of an international aviation delegation.
They also claimed the plane’s black boxes have been recovered and will be handed over to the International Civil Aviation Organisation.
The UK is supporting an Australian attempt to secure a UN security council (UNSC) resolution, which would demand “safe, full and unfettered access to the site” and for the bodies to be handled with respect and dignity.
A No 10 source said Russia had blocked an attempt to agree a press statement by the UNSC and the Government was “realistic” about the prospect of success in getting a resolution through without it being vetoed.
Meanwhile the US set out the evidence it claimed it had for Russian separatists being behind the atrocity.
Secretary of state John Kerry said it was “pretty clear” that an SA-11 missile system had been transferred by Russia into the hands of the separatists.
“There’s a build-up of extraordinary circumstantial evidence,” he said.
“We picked up the imagery of this launch. We know the trajectory. We know where it came from. We know the timing, and it was exactly at the time that this aircraft disappeared from the radar.
“We also know from voice identification that the separatists were bragging about shooting it down afterwards.”
Mr Kerry’s British counterpart Philip Hammond warned that Russia would be seen as a “pariah state” if it did not act responsibly.
In a series of broadcast interviews he accused Russia of “obfuscation and obstruction” and said Mr Putin could “snap his fingers” and allow a proper investigation to take place at the crash site, but that had not been done.
He warned Russia could become a “pariah state” if it did not behave properly on the international stage.
Mr Hammond, who has chaired a series of meetings with Whitehall officials including representatives of the intelligence agencies, said the evidence available about those who were behind the atrocity was not yet strong enough to stand up in court, but it would “lead the reasonable person to the unavoidable conclusion that this was a missile fired from rebel-held territory, almost certainly a missile supplied by the Russians”.
He acknowledged London could be affected by any restrictions on the flow of Russian money, and insisted other European countries would also have to take some of the pain of a more punitive sanctions regime – which could potentially include EU-wide restrictions on arms exports, a step the UK has already taken.
Mr Hammond said: “We have tools in our toolbox, we have levers which we can apply to Russia. We can inflict damage on the Russian economy.”
He acknowledged that “financial sanctions will have some impact on London” and insisted that as part of the package of sanctions “the pain has got to be shared across the European Union”.
French arms sales and German dependence on Russian fossil fuels have been seen as possible barriers to tougher sanctions.
Mr Hammond said one option that would be discussed was “broadening the number of individuals who are subject to sanctions to include the so-called crony group around president Putin”.
A No 10 source said the UK would expect additional names to be added to the list of Russian subject to travel bans and asset freezes under the existing criteria for EU sanctions.
These could include “entities” – firms or organisations – as well as individuals involved in supporting efforts to destabilise Ukraine.
But there will also be a push to extend the scope of the sanctions, to allow those who are influencing or supporting the “Russian regime” to be targeted for sanctions, meaning oligarchs within Mr Putin’s inner circle could be named.
The source said: “Our approach to Putin will be guided by how best do we think we will persuade him to engage with us and with the separatists to turn this around and find a more peaceful, different outcome.”