The walkouts followed the firing of a senior Serbian police officer in northern Kosovo after he refused to change his vehicle plates to Kosovo-issued ones as required by a rule change. Pristina’s decision has sparked controversial issues surrounding Kosovo’s sovereignty, especially among the Serb minority, which lives mainly in the north. Many of Kosovo’s Serb minority want the former Serb province to be part of Serbia, not independent. They do not recognize the authority of Pristina, the capital and center of power of Kosovo. Serbia itself has never recognized Kosovo’s independence. The sign change went into effect Tuesday. However, Kosovo authorities said enforcement would be gradual. In the next three weeks, Serbs will receive a warning if they keep their old Serbian license plates. Fines will follow for the next two months and then until April 31 they will only be able to drive with temporary local plates. A Serb government minister, 10 parliamentarians and police and judiciary officials have all resigned in the four Serb-dominated regions. Senior police officers symbolically removed their uniforms after a meeting on Saturday. The effect of the mass resignation was unclear. Kosovo Prime Minister Albin Kurti called on Serbs “not to become victims [to] political manipulations and geopolitical games,” urging them to stay with the state. Kurti claimed that Serbian President Aleksandar Vucic has been lying to US and European envoys who have tried to resolve their tensions, and “frequently meets and coordinates with the Russian ambassador in Belgrade.” “It is not a democratic country, Serbia is becoming a tool of the Kremlin,” Kurti posted on social media. Vucic said Serbia is determined to vigorously defend its vital national interests. Tensions came to a head this summer after Serbia and Kosovo refused to recognize each other’s identity documents and vehicle number plates. Kosovo Serbs in the north set up barricades, sounded air raid sirens and fired guns into the air. In August, the EU and the US helped negotiate a solution to the travel document problem, putting a temporary reprieve on the situation. The European Union has told Kosovo and Serbia they must normalize relations if they want to join the bloc. Brussels and Washington have recently stepped up efforts to mediate tensions, worried that concerns over the Ukraine war and Serbia’s close ties to Russia could spark a conflict between Serbia and Kosovo. Kosovo became independent in 2008. Its sovereignty is recognized by the US and 100 UN members. Serbia has appealed to Moscow and China in its bid to recapture the former province. Belgrade lost control of Kosovo in 1999 after NATO bombed the country to stop a brutal crackdown on Albanian separatists.