LUXEMBOURG — The European Union signed a pre-membership pact with Serbia, seeking to bolster pro-Western forces against surging nationalist parties in next month’s election.

The 27-nation bloc painted yesterday’s move as an act of political symbolism that will withhold the pact’s benefits from Serbia until it arrests the most wanted war-crimes suspects from the violent breakup of Yugoslavia in the 1990s.

Serbia’s “future is in Europe,” the Dutch foreign minister, Maxime Verhagen, said before the accord was signed at an E.U. meeting in Luxembourg. “But let me be clear: There will be no agreement that would undermine the principle that Serbia has to fully cooperate with the Yugoslavia tribunal” that tries those accused of war crimes.

Serbia, with 7.5 million people, has been encircled by pro-Western republics since Yugoslavia fell apart. Haunted by the wars of the 1990s, it has been the slowest ex-Yugoslav republic to pursue E.U. membership.

“It’s a strong signal for Serbia to join us, to come to the E.U.,” said the chairman of today’s meeting, Foreign Minister Dimitrij Rupel of Slovenia, which in 2004 became the only former Yugoslav republic so far to join the bloc.

With polls showing an edge for nationalists before the May 11 parliamentary elections, Serbia’s pro-Western parties had pressed the E.U. to sign the long-delayed agreement to persuade voters to put Serbia on the E.U. path.

President Tadic of Serbia dismissed concern that the signature ceremony would be an empty promise to Serb voters, predicting it will unleash a wave of foreign investment.

As Serbia grapples with “the burden of the destruction of war and war crimes,” the E.U. pact “will write a new, happier page in our joint history,” Mr. Tadic told reporters in Luxembourg.

Serbia’s nationalists say pro-Western politicians share the blame for the breakaway of the Serb province of Kosovo, which declared independence in February with E.U. and American backing.