President Aleksandar Vucic’s statement comes after shootout kills four people and fuels tensions in the volatile Balkan region.

“A campaign of lies … has been launched against our Serbia,” Serbian President Aleksandar Vucic said in a video posted on Instagram on Sunday.

“They have lied a lot about the presence of our military forces …. In fact, they are bothered that Serbia has what they describe as sophisticated weapons.”

Earlier this week, the United States urged Belgrade to pull its forces back from the border with Kosovo after detecting what it called an “unprecedented” Serbian military build-up.

Serbia deployed sophisticated tanks and artillery on the border after deadly clashes erupted at a monastery in northern Kosovo last week, the White House warned. The European Union also expressed similar concerns.

Kosovo’s government on Saturday said it was monitoring the movements of the Serbian military from “three different directions”. It also urged Serbia to immediately pull back its troops and demilitarise the border area.

Clashes last week erupted when heavily armed Serb gunmen ambushed a patrol a few miles from the Serbian border, killing a Kosovo police officer.

Several dozen assailants then barricaded themselves in an Orthodox monastery, sparking an hour-long firefight in which three gunmen were killed and three were arrested.

Kosovo’s government has accused Belgrade of backing the operation.

Serbia denied Kosovo’s allegations that it trained the group of some 30 men who opened fire on police officers.

Kosovo also said it was investigating possible Russian involvement in the violence. Serbia is Russia’s main ally in Europe, and there are fears in the West that Moscow could try to stir trouble in the Balkans to avert attention from the war in Ukraine.

The clash was one of the worst since Kosovo broke away from Serbia in a bloody war in 1998-99 and declared independence in 2008, prompting NATO to announce it would beef up a peacekeeping force stationed in the country.

Vucic has several times in recent months raised the combat readiness level of Serbian troops on the border with Kosovo.

Serbia also has been reinforcing its troops with weapons and other equipment mainly purchased from Russia and China.

“We will continue to invest in the defence of our country but Serbia wants peace,” the Serbian leader said on Sunday. “Everything they said they made up and lied, and they knew they were making up and lying.”