White House National Security Council Strategic Communications Coordinator John Kirby answered journalists' questions during his daily online press conference.

Stating that they have observed Serbia's "unprecedented" military buildup on the Kosovo border, Kirby said, "We believe this is a highly destabilizing development."

"We urge Serbia to withdraw these forces from the border and contribute to de-escalation," Kirby said.

Stating that the US is in close contact with Serbia and Kosovo, as well as NATO and its European allies, Kirby said that the incident in northern Kosovo on September 24, which resulted in the death of a policeman in Serbian gunfire, should be investigated.

Kirby stated that about 30 people were involved in the attack and underlined that the criminals should be brought to justice.

Stating that the weapons seized in Kosovo pose a threat not only to personnel in the region but also to international personnel, including NATO troops, Kirby said that they will continue to closely monitor the situation.

"In light of current developments, NATO's Peacekeeping Force (KFOR) presence in Kosovo will be strengthened," Kirby said.

Calling for restraint on all sides, Kirby pointed out that diplomatic efforts will continue.

Vucic: "Our meeting (with Blinken) was not easy"

Serbian President Aleksandar Vucic spoke by phone with US Secretary of State Antony Blinken.

"Our conversation was not easy," Vucic told reporters after the phone call, noting that they touched on what happened in northern Kosovo on September 24.

Vucic said, "Blinken talked about possible sanctions against Serbia if it does not act appropriately. I told him that the United States is a big and powerful country. I expressed my opposition to this," Vucic said.

Events in northern Kosovo on September 24

Months of tension in northern Kosovo escalated again on the morning of September 24 when armed Serbs blocking the road in Banjska, a predominantly Serbian town, opened fire and killed a policeman.

Kosovo Prime Minister Albin Kurti announced that a group of about 30 heavily armed men, whom he described as a "professional terrorist/criminal organization", attacked Kosovo police and blamed Serbia for the incident.

The armed group, which took refuge in and around the Banjska Monastery, was surrounded by Kosovo police and clashes continued throughout the day in the region.

Kosovo Interior Minister Xhelal Swedishla announced that at least 3 attackers were killed in the operation organized in Banjska and 6 people were detained, including 2 attackers and 4 people who supported the attack through radio connections.

Serbian President Vucic confirmed that the incident that resulted in the death of a Kosovo policeman was carried out by Kosovo Serbs.