By Fatos Bytyci MITROVICA, Kosovo (Reuters) -The United States, NATO and European Union urged maximum restraint in the north of Kosovo, where authorities closed a third border crossing on Wednesday amid an escalating dispute with local Serbs over its 2008 independence. For over 20 years, Kosovo has been a source of tension between the West, which backed its independence, and Russia, which supports Serbia in its efforts to block Kosovo’s membership of international organisations including the United Nations. “We call on everyone to exercise maximum restraint, to take immediate action to uncondi…