PRISTINA (Reuters) - The United Nations cautioned Serbia on Wednesday against holding its May local elections among Serbs in newly independent Kosovo, saying the results would not be considered valid.

"They are in violation of (U.N. Security Council Resolution) 1244 ... and will have no legal validity," said a spokesman for the U.N. mission that has run Kosovo since the 1998-99 war and pullout of Serb forces.

Serbia holds parliamentary and local elections on May 11, and plans to conduct both in Kosovo, its former province which declared independence with Western backing on Feb 17.

Ethnic Albanians make up 90 percent of Kosovo's 2 million people. Backed by Russia, Serbia and Kosovo's 120,000 remaining Serbs reject the secession.

The U.N. mission has indicated it has no objection to Kosovo Serbs voting in the parliamentary election as they have done before, but Kosovo's ethnic Albanian leaders say local polls represent a threat to the sovereignty of the new state.

U.N. spokesman Alexander Ivanko said the mission had received a request from Belgrade to hold the elections, and was drafting a response.

U.N. police and NATO peacekeepers would have difficulty preventing Serbs from voting in the local elections without resorting to force.

But U.N. officials have floated the idea of letting them vote, and refusing to recognize the results.

Serbia lost control over Kosovo in 1999, when NATO bombs drove out Serb forces to halt the killing and ethnic cleansing of Albanians in a two-year counter-insurgency war.

Serbia is now strengthening a network of Kosovo Serb parallel structures, telling Serbs to boycott the new state and deepening Kosovo's de facto partition.

(Reporting by Matt Robinson; editing by Stephen Weeks)