Unknown attackers threw two grenades at NATO barracks at the tense border between Kosovo and Serbia early Tuesday but there were no casualties, NATO troops said.

"At 4.00 am this morning (02:00 GMT Tuesday) two hand grenades were thrown at the KFOR Compound ... at the Brnjak crossing point," in northern Kosovo, the NATO-led KFOR mission said in a press release.

Voters across Serbia headed to the polls Sunday to elect a new president in a run-off with the incumbent pro-European Boris Tadic set for a victory over nationalist challenger Tomislav Nikolic.

Surveys published just ahead of the final round of the presidential election showed Tadic, 54, leading with 58 percent, while Nikolic had 42 percent.

Serbia's presidential election run-off this weekend will be a "referendum on the EU", the party of pro-European incumbent Boris Tadic, the frontrunner, said on Thursday.

Tadic faces nationalist challenger Tomislav Nikolic on Sunday.

Former Bosnian Serb army commander Ratko Mladic went on trial Wednesday accused of carrying out a brutal campaign of ethnic cleansing and Europe's worst massacre since World War II.

Mladic's trial opened at the Yugoslav war crimes court in The Hague, also watched in a live broadcast in Sarajevo by widows and other relatives of victims of the 1995 massacre at Srebrenica where almost 8,000 Muslim men and boys were allegedly murdered by Mladic's forces.

Former Bosnian Serb army commander Ratko Mladic went on trial Wednesday accused of carrying out a brutal campaign of ethnic cleansing, including Europe's worst atrocity since World War II.

"Ratko Mladic assumed the mantle of the criminal goal of ethnically cleansing Bosnia," prosecutor Dermot Groome told the judges at the Yugoslav war crimes court in The Hague.

Serbians vote Sunday in elections billed as crucial for their EU future by center-left President Boris Tadic, who faces a populist challenger in a campaign focused on jobs and the economy.

For Serb voters, it is the first election in decades to focus on economic issues rather than Balkan strife that led to international sanctions against Serbia.

Germany and Austria said Saturday they are sending some 700 more troops to Kosovo to help prevent unrest in the north of the breakaway province ahead of elections in neighboring Serbia on May 6.

Germany's defense ministry said 550 soldiers would be deployed at the request of the NATO-led KFOR peacekeeping force, while Austria's defense ministry said 150 troops would join the Kosovo Operational Reserve Force.

An ethnic Albanian from Kosovo was killed and his wife and four children wounded Sunday in a bomb explosion at their home in Kosovska Mitrovica, police said.

The bomb was placed overnight on the window ledge of the victims' home, in the Serb-majority northern part of town, police added.

Serbian President Boris Tadic on Thursday sent an official letter of resignation to the parliament to clear the way for joint presidential and parliamentary elections, Tanjug news agency reported.

Parliamentary speaker Slavica Djukic Dejanovic now has until Saturday to call presidential elections on the same day as the parliamentary, regional and local vote already scheduled on May 6.

Serbian President Boris Tadic said on Wednesday that he will step down to clear the way for simultaneous parliamentary and presidential elections on May 6.

"I decided to shorten my mandate to allow presidential elections to be held on May 6. I will be a candidate," RTS state television quoted Tadic as saying.