Vladimir Putin has been warned that he is "practically in a state of war with Europe" as EU leaders mull tougher sanctions over the Ukraine crisis.
Lithuanian president Dalia Grybauskaite delivered the dramatic message to Moscow as she arrived in Brussels for a summit.
Meanwhile, David Cameron has insisted there must be "consequences" if an estimated 1,000 Russian troops are not withdrawn from the east of the country.
And outgoing European commission president Jose Manuel Barroso said the situation was in danger of reaching "the point of no return".
The comments came after Nato released images apparently showing Russian forces and an array of heavy weaponry in Ukraine.
However, Mr Putin has denied that his forces are in Ukraine - and upped his own rhetoric by reminding critics that Russia has nuclear weapons.
Arriving at the summit in Brussels, Ms Grybauskaite - whose own country used to be part of the Soviet Union - suggested EU states should be supplying Kiev with military equipment.
"It is the fact that Russia is in a war state against Ukraine," she told reporters in English.
"That means it is in a state of war against a country which would like to be closely integrated with the EU. Practically Russia is in a state of war against Europe.
"That means we need to help Ukraine to ... defend its territory and its people and to help militarily, especially with the military materials to help Ukraine to defend itself because today Ukraine is fighting a war on behalf of all Europe."
Mr Cameron, who earlier held talks with Ukraine president Petro Poroshenko at a nearby hotel, said: "We have to address the completely unacceptable situation of having Russian troops on Ukraine soil.
"Consequences must follow if that situation continues and we will be discussing that as well today."
At a joint press conference with President Poroshenko this morning, Mr Barroso insisted it was not too late to find a political solution.
But he added: "We are in a very serious, I would say, dramatic situation... where we can reach the point of no return.
"If the escalation of the conflict continues, this point can come."
He added: "Russia should not underestimate the European Union's will and resolve to stand by its principles and values."
Mr Poroshenko said his presence at the summit was an important demonstration of solidarity with his country.
He said he still hoped tensions could be eased, after meeting Mr Putin in Minsk earlier this week.
"The most important thing now is peace," he said.
"Today we are talking about the fate of Ukraine, tomorrow it could be for all Europe."
EU leaders were originally meeting to decide on a new European Council President and High Representative, but those discussions are likely to be overshadowed by the burgeoning crisis.
The Prime Minister met new Commission President Jean-Claude Juncker - whose appointment he unsuccessfully tried to block - this morning to try to persuade him to hand Britain's nominee, Lord Hill of Oareford, a top economic portfolio.
However, the full 28 commission posts are not expected to be finalised tonight.
Mr Cameron is to suggest closer alignment between EU sanctions and those imposed by the US and Canada - raising the prospect of measures against specific Russian firms in sectors such as banking and energy.
After warning yesterday that extremism in Iraq and Syria poses a greater danger to Britain than al Qaida, the premier will also urge more co-ordinated European action to track jihadists.
The UK wants to revive a directive that would enable police and security services across the EU to share passenger records.
National leaders had signed off the arrangements - but they have stalled in the European Parliament after MEPs expressed concern about civil liberties and privacy.
Mr Cameron said: "Today in Brussels is an opportunity to talk with other EU leaders and make sure we all co-ordinate to stop people travelling Iraq and Syria, to stop radicalisation, to confront extremism."
Polish prime minister Donald Tusk is widely expected to secure the role of European Council President in succession to Herman van Rompuy, while Italian foreign minister Federica Mogherini is set to take over from Baroness Ashton as High Representative.
A UK government source said specific sanctions against Russia are not due to be agreed, as not enough work has been done on the details.
Mr Cameron is hoping that the leaders will lay down a "marker" as to what is necessary to avoid further punitive measures, which would probably focus on individuals close to Mr Putin, and the energy, defence and financial sectors.
It is understood Britain does not support supplying arms to Ukraine, with the source echoing Mr Poroshenko's view that there is "not a military solution" to the crisis.
Alongside restating its support for sharing passenger information to tackle the threat from Islamist extremists, the council is expected to pledge help to improve airport security and design in less developed countries.
Mr van Rompuy confirmed on Twitter that Mr Tusk would be succeeding him, and Ms Mogherini will be High Representative.
Speaking to journalists as he left the summit, Mr Cameron said the EU leaders had taken "some important steps".
"It is totally unacceptable that there are Russian soldiers on Ukrainian soil. We have now set out a timetable for further sanctions that could be, in the text, significant steps.
"It's a deeply serious situation and we have to show real resolve, real resilience in demonstrating to Russia that if she carries on in this way the relationship we have between Europe and Russia, Britain and Russia, America and Russia will be radically different in the future.
"We cannot go on like this. We know from European history the danger of the territory of a nation state being threatened and undermined in this way."