David Cameron is to push for harsher sanctions against Russia after it was accused of deploying more than 1,000 troops and an arsenal of weapons into Ukraine.
The Prime Minister will urge fellow European leaders to ratchet up the pressure when they meet for a summit in Brussels today.
The gathering comes after Nato released images showing the extent of Russian forces on the ground in the east of the country.
UK Government sources said they believed the separatists now had an array of heavy weaponry supplied by President Vladimir Putin, including 100 battle tanks, 100 artillery pieces, anti-tank weapons and missile launchers.
US President Barack Obama said the satellite pictures made it "plain for the world to see" that Moscow had "deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine".
"The violence is encouraged by Russia. The separatists are trained by Russia, they are armed by Russia, they are funded by Russia," he said. The Russian tanks filmed rumbling through Ukraine were "a continuation of what's been taking place for months now", he added.
Nato Secretary-General Anders Fogh Rasmussen insisted that the alliance would "fully respect" any membership bid by Kiev.
Ukrainian Prime Minister Arseny Yatseniuk has said he will ask parliament to abandon the country's non-aligned status and seek to join - a move that Russia has previously dismissed as unacceptable.
Mr Rasmussen said: "Despite Moscow's hollow denials, it is now clear that Russian troops and equipment have illegally crossed the border into eastern and south-eastern Ukraine. Russian forces are engaged in direct military operations inside Ukraine."
Meanwhile, Mr Putin issued a statement calling on pro-Russian separatists to avoid "senseless deaths" by releasing Ukrainian soldiers who have been surrounded. He is thought to have been referring to fighting outside the strategic town of Ilovaysk, east of Donetsk.
"I'm calling on insurgents to open a humanitarian corridor for Ukrainian troops who were surrounded in order to avoid senseless deaths," Mr Putin said. He did not address the claims about military presence. The crisis is to be discussed by EU leaders in Brussels tonight. Mr Cameron is expected to urge counterparts to demonstrate political will to ratchet up the sanctions regime.
He will call for closer alignment between EU sanctions and those imposed by the US and Canada - raising the prospect of measures against specific firms in sectors such as banking and energy.
Several EU foreign ministers accused Russia of invading eastern Ukraine. "We have to be aware of what we are facing: We are now in the midst of the second Russian invasion of Ukraine within a year," said Swedish foreign minister Carl Bildt, referring to Russia's annexation of Ukraine's Crimean Peninsula in April. "We see regular Russian army units operating offensively on the Ukrainian territory against the Ukrainian army. We must call a spade a spade."
All options except military action will be considered to punish Russia for pursuing "the wrong path," said Luxembourg's foreign minister Jean Asselborn.
"The EU should be ready to move forward with possible new measures against Russia because the situation is still getting worse," said Estonian foreign minister Urmas Paet.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules hereComments are closed on this article