The United States called on Belgrade to pull its forces back from the border with Kosovo on Friday (Sep 29) after detecting what it called an unprecedented Serbian military build-up. Serbia had deployed sophisticated tanks and artillery on the frontier after deadly clashes erupted at a monastery in northern Kosovo last week, the White House warned. The violence in which a Kosovo policeman and three Serb gunmen were killed marked one of the gravest escalations for years in Kosovo, a former Serbian breakaway province.