The walkouts followed the dismissal of a senior Serb police officer in northern Kosovo after he refused to change his vehicle’s license plates to ones issued by Kosovo as required by a rule change.

Pristina’s decision has ignited contentious issues around Kosovo’s sovereignty, especially among its Serb minority, who mainly live in the north.

Many of Kosovo’s Serb minority wants the former Serb province to be part of Serbia, not independent. They do not recognize the authority of Pristina, the Kosovar capital and center of power.

Serbia itself has never recognized the independence of Kosovo. The number plate change came into effect on Tuesday. But Kosovo authorities said enforcement would be gradual.

Over the coming three weeks, ethnic Serbs will be warned if they keep their old Serbian license plates. This will be followed by fines in the next two months, and then until April 31 they can only drive with temporary local plates.

An ethnic Serb government minister, 10 MPs and officials in the police and judiciary all resigned in the four areas dominated by Serbs. Senior police officers symbolically took off their uniforms after a meeting Saturday.

The effect of the mass resignation was unclear.

Kosovar Prime Minister Albin Kurti called on Serbs “not to fall prey [to] political manipulations and geo-political games”, urging them to stick with the state.

Kurti claimed that Serbian President Aleksandar Vucic was lying to US and European envoys, who have tried to resolve their tensions, and “often meets with and coordinates with the Russian ambassador to Belgrade.”

“Not being a democratic country, Serbia is becoming a Kremlin tool,” Kurti posted on social media.

Vucic said that Serbia is determined to strongly defend its vital national interests. Tensions bubbled to the surface this summer after Serbia and Kosovo refused to recognize each other’s identity documents and vehicle license plates.

Kosovo Serbs in the north put up roadblocks, sounded air raid sirens and fired guns into the air.

In August, the EU and US helped negotiate a solution to the travel documents problem, giving temporary reprieve to the situation. The European Union has told Kosovo and Serbia they must normalize relations if they want to join the bloc.

Brussels and Washington recently stepped up efforts to mediate the tensions, worried that concerns over the Ukraine war and Serbia’s close ties with Russia could stir up conflict between Serbia and Kosovo.

Kosovo became independent in 2008. Its sovereignty was recognized by the US and 100 members of the UN. Serbia has appealed to Moscow and China in its attempt to retake the former province.

Belgrade lost control over Kosovo in 1999 after NATO bombed the country to stop its brutal crackdown against ethnic Albanian separatists.