WRAPUP 5-EU and Serbia sign pact on closer ties before polls

* EU signs key pact with Serbia

* Compromise aims to keep up pressure on war crimes suspects

* No agreement on EU-Russia mandate

(Releads with Tadic, reactions and Dutch, British)

By David Brunnstrom and Mark John

LUXEMBOURG, April 29 (Reuters) - Serbia signed a long-delayed pact on closer ties with the European Union on Tuesday, a first step towards eventual membership, days before a parliamentary election.

The accord had been held up over Serbia's failure to arrest and hand over key war crime indictees such as wartime Bosnian Serb general Ratko Mladic, charged with genocide over the 1995 Srebrenica massacre of some 8,000 Bosnian Muslims.

Under a compromise proposed by the Netherlands and Belgium, the 27 EU states agreed not to ratify the pact nor give Serbia its trade or aid benefits until all agree that Belgrade is fully cooperating with the U.N. war crimes tribunal.

But the EU hopes signing the pact will bolster President Boris Tadic's pro-Western reformers in the May 11 election. Polls currently show nationalists have a slight lead, boosted by anger at the Western-backed secession of Kosovo in February.