Serbia has raised the combat readiness of its troops on the border with Kosovo.
This comes amid increasing tensions with its breakaway former province, the Serbian defence minister said.
Ethnic Serbs in Kosovo were blocking the border for a fourth straight day to protest about a decision by Kosovo authorities to start removing Serbian licence plates from cars entering the country.
There are fears the latest incidents could unleash deeper tensions between the two Balkan foes.
Kosovo has deployed its special police force to the predominantly ethnic Serb-populated area of Kosovo to enforce the new rule. Serbia itself has for years been taking off registration plates from Kosovo-registered cars entering Serbia and Kosovo officials say the new rule is a tit-for-tat measure.
Serbian media reported that three Serb protesters had been severely beaten by Kosovo policemen, a claim vehemently denied by Kosovo authorities.
Belgrade described the alleged incident as use of “brutal force”. Serbian President Aleksandar Vucic said he was “concerned” and warned that violence was unacceptable.
“We have made clear what are the boundaries by which we will tolerate violence by (Kosovo Prime Minister) Albin Kurti and his special forces,” state Serbian RTS TV quoted Vucic as saying.
“Both the Europeans and Americans know that. We have said very precisely what our next moves will be, and in which order they will be made,” he said.
Serbia is unlikely to intervene militarily in Kosovo, where thousands of international peacekeepers have been deployed since the 1998-99 war that stopped a bloody Serb crackdown against ethnic Albanians.
Still, Serbian Defence Minister Nebojsa Sefanovic, inspecting troops on the border, said Serbia stands ready to protect its citizens.
“Our army is not provoking, but it’s ready to protect its people,” Mr Sefanovic said.
The war in Kosovo ended after a NATO intervention, and Kosovo declared independence in 2008. It has been recognised by the US and other Western nations, but not by Serbia and its allies Russia and China.
The European Union and the US have urged Kosovo and Serbia to exercise restraint.