EU leaders are meeting in Brussels to discuss the crisis in Ukraine, threatening to impose fresh sanctions against Russia.
EU foreign policy chief Catherine Ashton accused Russia of "direct aggression" in eastern Ukraine.
Lithuanian President Dalia Grybauskaite said Russia was "practically in a war against Europe".
Russia denies that its forces are backing rebels, who have been gaining ground on Ukrainian forces.
Baroness Ashton said there was "deep concern" over "direct aggression by Russian forces". She called on Russia to stop the flow of arms, equipment and personnel into Ukraine.
As she arrived at the talks in Brussels, Ms Grybauskaite said: "We need to support Ukraine, and send military materials to help Ukraine defend itself. Today Ukraine is fighting a war on behalf of all Europe."
UK Prime Minister David Cameron said the EU faced "a completely unacceptable situation of having Russian troops on Ukrainian soil. Consequences must follow if that situation continues".
French President Francois Hollande said the Ukraine crisis was the biggest since the end of the Cold War.
He said: "What's happening in Ukraine is so serious that the European Council will be obliged to react by increasing the level of sanctions if things remain as they are."
The EU and the US have already imposed sanctions against dozens of senior Russian officials, separatist commanders and Russian firms accused of undermining Ukrainian sovereignty.
However, the BBC's Chris Morris in Brussels says there are still divisions within the EU on how to deal with the Ukraine situation.
Finnish Prime Minister Alexander Stubb said the "jury is still out" on whether sanctions had worked, adding: "We need to find a ceasefire, a peace plan."
European Commission President Jose Manuel Barroso warned Russia that the EU was ready to "stand by its principles" and called for a political solution before the crisis reached a "point of no return".
He was speaking after talks with Ukrainian President Petro Poroshenko, who is also due to attend the EU leaders' summit.
Mr Poroshenko said Ukraine was a victim of "military aggression and terror" involving "thousands of foreign troops and hundreds of foreign tanks".
'Ordered to retreat' Government forces have lost ground in recent fighting.
A Ukrainian military spokesman said on Saturday that Russian tanks had attacked the town of Novosvitlivka near Luhansk and "destroyed virtually every house".
Spokesman Andriy Lysenko said troops had been ordered to retreat from Novosvitlivka.
Troops are also reportedly trying to evacuate Ilovaisk in the Donetsk region. It has been surrounded by rebels.
Rebel leader Alexander Zakharchenko told the Russian News Service radio station a new offensive was being planned to create a corridor between Donetsk and Luhansk.
In south-eastern Ukraine, people have been leaving the port city of Mariupol, after advancing rebels captured Novoazovsk to the east.
Western and Ukrainian officials say this offensive has been substantially helped by Russian regular troops, opening a new front. Russia denies the accusation.
Some 2,600 people have died in fighting in the Donetsk and Luhansk regions since April.
The conflict erupted following Russia's annexation of Ukraine's southern Crimea peninsula a month before.