European Commission President Jose Manuel Barroso has told Russia that any problems it has with an individual EU state are problems with the whole bloc.
Speaking at an EU-Russian summit, Mr Barroso said the EU was based on principles of solidarity. The summit, near the city of Samara, was marred by Moscow's rows with countries including Estonia and Poland.
Disputes between Moscow and Brussels have also arisen over the status of Kosovo, energy supplies and trade.
German Chancellor Angela Merkel, whose country holds the EU's rotating presidency, expressed concern at the difficulties Russian opposition activists were reporting in getting to the summit venue.
Moscow police prevented former world chess champion Garry Kasparov and other opposition leaders from flying to attend a protest rally. Several foreign journalists were also reportedly prevented from travelling.
Russian President Vladimir Putin, for his part, condemned the treatment of ethnic Russians in the Baltic states.
In a break with previous practice, no joint declaration was prepared before the summit at Volzhsky Utyos government resort.
Polish veto
"We had an occasion to say to our Russian partners that a difficulty for a member state is a difficulty for the whole European community," Mr Barroso said at the news conference.
"The Polish problem is a European problem. The Lithuanian and Estonian problems are also EU problems. "It is very important if you want to have close co-operation to understand that the EU is based on principles of solidarity," Mr Barroso said.
He was referring a Russian trade dispute with Poland, the cutting of Russian oil supplies to Lithuania in a separate row, and a bitter dispute over the treatment of ethnic Russians in Estonia.
Poland has imposed a veto on any new strategic partnership accord between the EU and Russia. The veto - now also supported by Lithuania - follows Russia's decision last year to block meat imports from Poland over apparent food safety issues.
"We believe there is no reason for a (Russian) ban on Polish meat imports," Mr Barroso told President Putin.
'Unacceptable violations'
Another major factor in the deterioration of relations has been Estonia's removal last month of a World War II monument to Red Army soldiers in central Tallinn.
The event sparked unrest by mostly ethnic Russians in Estonia, that left one person dead and more than 160 injured.
At the summit, Mr Putin accused Estonia - and also Latvia - of "unacceptable " violations of the rights of Russian-speakers.
He also condemned what he described as heavy-handed methods of Estonian police during the riots. "They didn't just disperse demonstrators. They killed one demonstrator. We demand that the criminals be brought to account," Mr Putin said.
The Kosovo issue also dominated the summit's agenda.
EU leaders have recently expressed alarm about Russian threats to veto a UN Security Council resolution proposing Kosovo's de facto independence from Serbia.
(BBC News, 18/05/2007)