The European Union insisted that Russia sign a key agreement on trade and other issues before the bloc's enlargement on May 1 to "avoid a serious impact on relations".

The EU's ties with Russia are governed by a Partnership and Cooperation Agreement (PCA). Brussels wants the PCA to apply automatically to Russia's relations with the 10 countries due to join in May - eight of which are former Soviet bloc states - from the day they join.

Moscow says this will cost it the trade and other privileges it enjoys with its closest neighbours. The EU argues that Russia may be disadvantaged in some markets, but will be much better off overall.

"The Council (of EU foreign ministers)... emphasises that the PCA has to be applied to the EU-25 without pre-condition or distinction by May 1, 2004," the council said in a statement.

"To do so would avoid a serious impact on EU-Russia relations in general. The EU is open to discuss any of Russia's legitimate concerns over the impact of enlargement, but this shall remain entirely separate from PCA extension."

Brian Cowen, foreign minister of Ireland, which holds the EU's rotating presidency, said a provisional deal could be reached by May 1, although an EU official said it would still need ratification by the European parliament and Russia's Duma.

"We've no reason to believe that we can't resolve this problem and we're attaching a priority to it and showing solidarity to acceding member states," Mr Cowen told a news conference. "The Russian Federation is aware of our position."

French Foreign Minister Dominique de Villepin said extending the PCA was "a position of principle", telling a briefing: "What was valid for the 15 is valid for the 25. Our Russian friends have got to understand this."

EU foreign policy chief Javier Solana said he thought the matter could be solved.

"We have a partnership with Russia which is strategic and I don't see any reason to think that we won't find a solution to that problem," he told reporters.

The EU has long had concerns over Moscow's war in Chechnya and on human rights, leading to frequent strains in relations.

But the wrangling over the PCA has added a new source of tension. EU officials have said the bloc could retaliate with sanctions if Russia applied trade terms on goods from new EU states that were different from those applied to the current 15.