JARINJE, Kosovo (Reuters) - NATO peacekeepers were being sent to newly independent Kosovo's border with Serbia on Tuesday to defend frontier posts under attack by Serbs who oppose its secession.

Serbs burned down one border post and were attacking a second, a Kosovo police spokesman said.

Kosovo police manning the post called for help from the NATO peacekeeping force, KFOR, which said it was stepping in.

"KFOR is going to intervene now," a force spokesman said. He declined to say which troops of the 35-nation, 17,000-stong force were being deployed.

The violence highlights the challenge facing an EU law enforcement mission preparing to deploy in the Albanian-majority territory which has been under U.N. administration for nearly nine years.

"Protesters have destroyed the border crossing post at Gate 1 in Jarinje," the Kosovo police spokesman said. "No one has been injured."

Protesters were also attacking a second post at Zubin Potok in Kosovo's Serb-dominated north, he said.

Police had taken shelter in a tunnel there as more than 1,000 protesters tried to tear it down, Kosovo police sources said.

"We asked NATO to send a helicopter to evacuate our officers," a police source told Reuters in Pristina.

KFOR forces in the district include French, Danish, Belgian and American units.

"The border crossing post at Jarinje is on fire and the mob has dispersed," one eyewitness said.

Albanian officers of the Kosovo police retreated and Serb officers crossed over into Serbia proper, police sources added.

Local Serbs backed by the Serbian government and Russia say the planned European Union supervisory mission to Kosovo, which will deploy 2,000 police and justice officials, is illegitimate and warn that its authority will not be accepted.

EU foreign affairs chief Javier Solana was expected in Kosovo later in the day to congratulate leaders on Sunday's independence declaration, recognized by most major Western powers but denounced by Serbia and Russia as illegal secession.

But angry Serb demonstrations and two nights of vandalism against vehicles and symbols of the international presence in Kosovo have thrown down a gauntlet to the incoming "EULEX" mission.

The EU expects to send the 2,000 police and justice officials to Kosovo to take over from the U.N. mission that has administered the province since NATO intervened in 1999 to end a Serb counter-insurgency war.

NATO had said on Monday conditions on the ground in Kosovo were quiet after its declaration of independence and there was no current need to reinforce its peacekeeping force.