More than 1,300 British troops are to take part in a major Nato exercise in Eastern Europe in a further show of strength by the Western alliance in the face of Russia's continuing "destabilisation" of Ukraine.

Speaking on a visit to Warsaw, Defence Secretary Michael Fallon, said that the UK would be sending a full battle group to take part in Exercise Black Eagle in Poland this autumn, in the largest British commitment to the region since 2008.

The announcement came as David Cameron and other key EU leaders agreed to press for a final deal on far reaching economic sanctions against Moscow targeting Russia's defence, energy and financial sectors.

In a conference call with US president Barack Obama, Mr Cameron, Germany's chancellor Angel Merkel, French president Francois Hollande and Italian prime minister Matteo Renzi, all accepted measures should be in place as "soon as possible", Downing St said.

A No 10 source said that they now hoped to see them push forward on the so-called "Tier 3" measures at a meeting tomorrow of EU ambassadors, although the source acknowledged that it would require agreement of all 28 nations.

"There was a clear sense that the European members states on the call would like to see agreement tomorrow, but until it is done it is not done. But we think it is important to make progress," the source said.

If agreed, the measures are expected to go beyond the existing travel bans and asset freezes against individuals and hit Russia's access to European capital markets and trade in the defence sector and sensitive technologies in the energy sector.

Earlier, Mr Fallon, on a joint visit to Poland with Foreign Secretary Philip Hammond, said that it was important to underline Nato's commitment to the collective security of its members at a time of heightened tensions in the region.

"It is right that Nato members and partners demonstrate our commitment to the collective security of our allies in Eastern Europe," he said.

"In particular, the commitment of a battle group to Exercise Black Eagle shows our sustained and substantial support to Nato's eastern border."

He said that the issue would also be high on the agenda when Nato leaders gather for their summit in Wales in September.

"We have a strong opportunity at the Nato summit to discuss how we will continue our response to Russia's illegal annexation of Crimea and its destabilisation of eastern Ukraine," he said.

"The UK is playing a central role and we are not ruling out further enhancements."

In all, Britain is to deploy 1,350 personnel and more than 350 armoured and other vehicles to take part in Exercise Black Eagle.

Since Russia's annexation of Crimea, the UK has deployed RAF Typhoon jets to the Nato Baltic air policing mission, as well as participating in smaller scale army exercises across Europe.

At the end of August light infantry troops from 1st Battalion, The Duke of Lancaster's Regiment, will take part in the US-led Exercise Sabre Junction, which is also taking place in Poland.

An international police team has failed for a second time to reach the site of the crashed Malaysian Airlines airliner amid continued fighting in the area between Ukrainian forces and pro-Russian separatists.

A team of Australian and Dutch police and forensic experts was unable to get beyond the town of Shakhtarsk, around 20 miles from the fields where the aircraft came down.

The government in Kiev, backed by the US, Britain, and other Western allies have blamed the separatists for bringing down the airliner in a surface-to-air missile strike - a claim denied by the separatists and their allies in Moscow.

Ukrainian security spokesman Andriy Lysenko said data from the aircraft's flight data recorders showed the plane suffered "massive explosive decompression" after it was hit by fragments he said came from a missile.

The data recorders have been sent to experts in Britain for examination.

The United Nations High Commissioner for Human Rights Navi Pillay said the downing of the aircraft may constitute a war crime and called for a thorough investigation into what happened.

Mr Cameron is set to meet the families of the some of the 10 British victims killed in the crash.

A No 10 spokeswoman said the leaders in the conference call agreed that the EU should bring forward a strong package of "sectoral sanctions" on Russia as soon as possible.

In a read-out of the call, she said: "(The leaders) agreed that the immediate priority must be to secure unrestricted access to the MH17 crash site, both to recover any remaining victims and to allow an unimpeded investigation of the site. They noted their support for Dutch and Australian efforts to secure such access.

"On the broader conflict in eastern Ukraine, they agreed that Russia has failed to take the steps necessary to de-escalate the crisis, such as ceasing support for the separatists; stopping the flow of weapons across the border; and using its influence to ensure the release of hostages.

"Indeed the latest information from the region suggests that even since MH17 was shot down, Russia continues to transfer weapons across the border and to provide practical support to the separatists.

"Leaders agreed that the international community should therefore impose further costs on Russia and specifically that ambassadors from across the EU should agree a strong package of sectoral sanctions as swiftly as possible.

"They agreed that the EU and United States should continue to work together to exert pressure on Russia to change course and to engage in a political resolution to the crisis before more innocent lives are lost."

Downing Street confirmed that Mr Cameron had invited the families of British victims of the MH17 crash to a meeting tomorrow.

A spokesman said: "The Prime Minister has invited the families of victims of MH17 for a meeting in Downing Street tomorrow to express his condolences for their tragic loss and to discuss the ongoing international response to the crash and any further support the Government can offer to families."