By Fatos Bytyci and Aleksandar Vasovic
MITROVICA/BELGRADE (Reuters) - Serbian police and NATO troops reinforced checkpoints on both sides of the Kosovo border on Thursday to deter further ethnic violence after a frontier post was burned down and a policeman killed.
"We will prevent extremists from Serbia going to Kosovo, we will aid international peacekeepers there in any way we can," Serbian police chief Milorad Veljovic told Reuters.
The border was quiet on Thursday after two days of violence in Kosovo's Serbian-populated north in which one ethnic Albanian policeman died and a border crossing was set on fire by hardline Serbian nationalists.
The troubles in Kosovo, which has a 90 percent ethnic Albanian majority, started after Pristina sent special police units on Monday to take control of northern border crossings and enforce a ban on imports from Serbia -- in retaliation for its block on Kosovo's exports in a dispute over customs regulations.
The police retreated after the violence but Kosovo Prime Minister Hashim Thaci said on Thursday he would send them back to all border crossings.
"Reciprocity (trade) measures with Serbia will be applied in all border crossings ... and this will continue until Serbia will change its position," Thaci told parliament.
Kosovo declared independence in 2008 in a move not recognized by Serbia and the 60,000 Serbs who live in northern Kosovo still consider Belgrade their capital. Another 40,000 Serbs live in enclaves in the rest of Kosovo.
Serbs cherish Kosovo as the historic heartland of their Orthodox Christianity and most are bitterly opposed to its independence.
On Thursday, the situation at the burned Jarinje border crossing was calm with U.S. troops from the NATO peacekeeping contingent in Kosovo in control and checking cars for weapons, a Reuters correspondent said.
German General Erhard Buehler, commander of NATO troops in Kosovo, said buildings at the border crossing, some 20 km (12 miles) north of the city of Mitrovica were heavily damaged.
"For security reasons there is high risk for people. Small private cars may cross the boundary at their own risk," Buehler said in a statement.
He was scheduled to meet a Serbian government negotiating team later in the day.
Peacekeepers, who still patrol Kosovo more than a decade after a 1998-1999 war between ethnic Albanians fighting for independence and Serb security forces, were also in control of another key border checkpoint.
Buehler also said KFOR would continue to deploy troops throughout Kosovo's north to safeguard security and freedom of movement.
Jean-Francois Fitou, France's ambassador in Pristina, told Reuters that "from the security point of view, the situation is not out of control but it is not good."
During Wednesday's attack on the Jarinje border post, Serbs threw firebombs and fired at members of NATO's KFOR peacekeeping force.
On Wednesday Thaci accused Belgrade of masterminding the violence, but President Boris Tadic said that the attacks were staged by hardline Serb nationalists opposed to Serbia's bid to mend ties with its former southern province.
"Violence, from any side, is not a way of solving problems," Serbian government spokesman Milivoje Mihajlovic told Reuters.
"For us, dialogue is the only way to solve problems in Kosovo...and reach historic reconciliation between Serbs and Albanians."
EU foreign policy chief Catherine Ashton said she had warned both Thaci and Tadic that violence would not be tolerated.
Belgrade wants to join the EU but must mend its ties with Kosovo to speed up the accession process. Pristina and Belgrade have started EU-moderated talks to improve trade, movement of people and issues like energy supplies, but negotiations have moved slowly.
The U.N. Security Council on Wednesday accepted a Serbian request for urgent closed-door consultations on Kosovo.
Serbia lost control of Kosovo in 1999, when NATO waged a 78-day bombing campaign to end Yugoslav leader Slobodan Milosevic's crackdown on ethnic Albanian rebels and a bloody campaign of ethnic cleansing.
(Additional reporting by Nebojsa Markovic; Editing by Barry Moody)