European Union foreign ministers Monday warned Russia of a "serious impact" on relations if Moscow fails to sign a new partnership accord before the EU enlarges deep into the former Soviet bloc in May.

The ministers told Russia to stop stalling on signing a new Partnership and Cooperation Agreement (PCA) to cover the 10 countries joining the European Union on May 1.

Describing the PCA as the "cornerstone" of EU-Russia relations, they warned Moscow in a statement to renew the accord "without pre-condition or distinction by May 1".

"To do so would avoid a serious impact on EU-Russia relations in general," it said.

Asked what the EU meant by a "serious impact" on relations, one diplomat said: "It's a threat of sanctions."

Irish Foreign Minister Brian Cowen, whose country holds the EU's rotating presidency, declined to be drawn on what steps the bloc might take against Russia.

But he added that the bloc was "optimistic that we can arrange for a provisional agreement to be signed before May 1".

"We've no reason to believe that we can't resolve this problem," he told a news conference.

The EU statement said renewing the accord was in Russia's own interest.

"The timely extension of the PCA will allow Russia to benefit from the many advantages accruing to it from EU enlargement, including in the trade field," it said.

The EU signed a PCA with Russia in December 1997, setting out the political, economic and trade frameworks of its relationship with the vast country for the next 10 years.

But the accord now needs to be adapted for the EU's enlargement to encompass eight former Soviet satellites -- the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia -- plus Cyprus and Malta.

Russia, for decades the dominant power in eastern Europe, has grudgingly accepted the enlargement as inevitable.

But it is still pressing for its demands to be met, such as visa-free access for its citizens to the new EU members.

In particular, Russia fears it will lose millions of dollars by being forced to pay higher trade tariffs on its exports after its former Warsaw Pact partners join the EU.

The EU, however, insists that renewal of the PCA should be a legal formality and is growing increasingly frustrated with the Russian stance.

"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," the foreign ministers' statement said.

Writing in the Wall Street Journal Europe Monday, the foreign ministers of Denmark and EU entrant Poland said it would be "inconceivable" if the PCA were not to be renewed before May.

"If Russia is not able to accept the enlargement of the European Union, Russia questions the basis of our strategic relationship," wrote Danish Foreign Minister Per Stig Moeller and his Polish counterpart Wlodzimierz Cimoszewicz.

The EU statement said the bloc wanted to build "a genuine strategic partnership with Russia based on equal rights and obligations, mutual trust and an open and frank dialogue".

Diplomats said the EU's reference to a "frank dialogue" with Russia included Chechnya, press freedom ahead of presidential elections next month, and Russian foot-dragging on the Kyoto Protocol on climate change.

EU leaders demanded a review of relations with Russia in December after the embarrassment of the EU's previous presidency, held by Italy, when Prime Minister Silvio Berlusconi flouted the common EU line by praising President Vladimir Putin's bloody campaign against rebels in breakaway Chechnya.

The EU and Russia are to hold their next bilateral summit in May, days after the EU's big bang enlargement.