The United States warned Serbia against its "unprecedented" military buildup near the border with Kosovo, highlighting its implications for regional stability.
The NATO military alliance said earlier it had authorized the deployment of additional peacekeeping forces in Kosovo after a battle last weekend between an unidentified Serb group and Kosovar police.
The Serbian military operation on the border includes an "unprecedented" deployment of artillery, tanks and troops, White House National Security Council spokesperson John Kirby said Friday.
"We call on Serbia to withdraw its forces from the border and continue to reduce tensions," he said in Washington.
Kirby said he could not say anything about Serbia's motive for the military build-up.
Kosovo seceded from Serbia in 1999 with NATO assistance after fighting an armed conflict. The country, which is mostly populated by ethnic Albanians, declared independence in 2008.
More than 100 countries recognize the state of Kosovo. Serbia, however, has refused to recognize Kosovo's independence and has demanded the return of what it considers a breakaway province.
Last Sunday, a 30-strong group of Serb insurgents took up position in the northern Kosovo village of Banjska and engaged in a firefight with local police.
Three Serbian attackers and a Kosovar policeman were killed.
Kosovar authorities described the Serbs as a heavily armed, uniformed, professional military or police force.
The government in Pristina assumes that neighboring Serbia trained, equipped, financed and sent the irregular militiamen to deliberately destabilize Kosovo.
The Serbian leadership denies the accusations but has so far failed to provide a conclusive explanation of the squad's origins and command responsibility.
Washington raised its concerns about the situation hours after NATO decided to send more soldiers to strengthen its KFOR peacekeeping troops in Kosovo.
The North Atlantic Council, the organization's main decision-making body, approved the deployment of additional forces to the small Balkan country, the alliance said Friday. It did not state how many additional soldiers were to be sent to the region.
Britain may be providing the reinforcements, dpa has learned.
The British Ministry of Defence said London would continue to make a "decisive contribution" to the protection force until at least 2026, in a statement a few months ago.
