EU leaders are to meet in Brussels to consider fresh sanctions against Russia following accusations that its troops are fighting inside Ukraine.
Russia denies that its regular forces are backing a rebel offensive in eastern Ukraine.
But ahead of the summit, EU officials said a clear message must be sent to Russia to de-escalate the crisis.
In Ukraine, a number of government troops remain encircled by the rebels in the Donetsk region.
Reports say the pro-Russian fighters have issued an ultimatum to the Ukrainian troops near the town of Ilovaisk to surrender by Saturday morning.
Some 2,600 people have died in fighting in the Donetsk and Luhansk regions.
The conflict there erupted in April following Russia’s annexation of Ukraine’s southern Crimea peninsula a month before.
In Brussels, the leaders of the 28-member bloc are also to decide who will fill two of Europe’s top jobs: president of the European Council and foreign policy chief to replace Herman Van Rompuy and Catherine Ashton.
Ukrainian President Petro Poroshenko is in Brussels to meet Mr Van Rompuy and Jose Manuel Barroso, the outgoing head of the Commission, ahead of the summit.
Mr Poroshenko is expected to press for tougher action against the Kremlin.
On Friday, German Foreign Minister Frank-Walter Steinmeier warned that the “already dangerous situation” in Ukraine had now entered “a whole new dimension”.
“The border infringements have intensified, and raised concerns that the situation is slipping out of control.
“This needs to stop, especially if we want to avoid direct military confrontation between Ukrainian and Russian military forces.”
He was referring to claims by Ukraine that regular Russian troops had entered Ukraine, capturing the south-eastern town of Novoazovsk.
Russia denies the accusation.
Mr Steinmeier’s sentiments were echoed by other EU foreign ministers:
- France’s Laurent Fabius spoke of “unacceptable” intervention by Russian troops in eastern Ukraine
- Sweden’s Carl Bildt said a clear message had to be sent to Russia: “We have to be aware of what we are facing: we are in the midst of the second Russian invasion of Ukraine within a year”
- The Netherlands’ Frans Timmermans said the presence of Russian soldiers in eastern Ukraine could not “remain unanswered”
But it remains unclear if a new round of sanctions will be adopted in Brussels.
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.
In late July, the EU also blacklisted some key economic sectors, prompting Russia to retaliate by banning food imports.
Russia’s energy minister has warned that the Ukrainian crisis could lead to a disruption of gas supplies to European countries this winter.
On Friday, Nato held an emergency meeting after releasing satellite images it said showed columns of Russian armed forces inside Ukrainian territory.
Secretary-General Anders Fogh Rasmussen called on Russia to “take immediate and verifiable steps towards de-escalation”.
Mr Rasmussen also indicated Nato could consider Ukraine’s application to join the alliance, shortly after Prime Minister Arseniy Yatsenyuk announced he was putting Ukraine on course for Nato membership.