PRISTINA, Serbia (Reuters) - The death toll from an attack by masked gunmen on a Serb cafe in Kosovo rose to six on Tuesday after two out of five youths injured died of their wounds, a source close to the local police said. Four Serb youths were killed instantly when two masked gunmen opened fire in the Cafe Panda in the centre of the western Kosovo town of Pec. The youngest was 15 years old. "Two of the five who were wounded died overnight," said the source, who declined to be named. The latest report brings Monday‘s total death toll to 37, after 31 ethnic Albanians were killed in clashes with Yugoslav security forces on Kosovo‘s border with Albania. The violence shattered a fragile two-month-old truce in the province and increased fears of a resumption of all-out fighting if a political settlement cannot be brokered soon. Initial reports had said there was just one gunman, but the Serbian-run Media Centre said later there were two. "According to the first results of the investigation, two masked terrorists opened fire from Chinese-made automatic weapons on the guests of the Pec cafe where high school youngsters gather," it said in a report. The gunmen were still at large, it added. One of the three wounded survivors was in a serious condition in hospital, the source said. A second was reported critical but stable, and the third was slightly injured. Pec is the main western town in the troubled Serbian province of Kosovo which is struggling to maintain a shaky ceasefire after eight months of bloodshed. International verifiers in Kosovo confirmed on Monday evening that at least people had died in the bar. The verifiers are in Kosovo to try to restore peace to the province, where the ethnic Albanian majority is demanding independence after a decade of harsh direct rule from Belgrade. At least 1,500 people have been killed by the fighting between Serbs and ethnic Albanians and a quarter of a million were driven from their homes before a partial Serb withdrawal, undertaken in October after NATO threatened airstrikes. U.S. Balkans envoy Richard Holbrooke returned to Yugoslavia on Tuesday for the first time since his October pullout deal with Yugoslav President Slobodan Milosevic to urge him not to break the truce and instead agree a compromise autonomy deal with the ethnic Albanians.