Serb security forces Thursday stormed the headquarters of Kosovo's ruling party, moments after ethnic Albanian separatists convened their clandestine parliament for the first time.

There was no official Serb explanation for the action by about 20 heavily armed riot police and 10 plainclothes officers, who entered the drab makeshift building in the heart of Kosovo's capital.They impounded all documents from the first session of the clandestine parliament, which is considered illegal by the Serbian auth-orities.

There was no violence or arrests, and the police withdrew after about 30 minutes.

Ethnic Albanians, who comprise 90 percent of Kosovo's 2 million people, held elections on March 22 for their 120-seat self-styled parliament and president.

Thursday's Serb police action was the first ever at the ruling Democratic League of Kosovo party headquarters, which also houses the office of their leader, Ibrahim Rugova. Rugova was inside the building during the raid.

"The police action wasn't brutal, but it was very swift," said Nekibe Kelmendi, the general secretary of the ruling party, which holds 90 percent of seats in the new parliament.

"They gave us a two-minute ultimatum to hand them the documents from the constitutional session, which we did," she said.

During Thursday's parliament session, Rugova took his presidential oath, pledging to "use all his powers for the formation and preservation of an independent Republic of Kosovo."

Kelmendi said the formation of the Kosovo parliament was "a historic act and the result of a new reality in the region which the international community will have to consider."

Serbia rejects any moves toward Kosovo's independence. Foreign powers have called for greater Kosovo autonomy, but also oppose independence of the troubled Serbian province.

More than 300 people have been killed since late February, when Serbian police launched a crackdown against ethnic Albanian separatists.

In Washington Thursday, a senior defense official said the military balance in Kosovo has changed so much that the United States and its NATO allies are reluctant to make any quick decision on using military force.