NATO's secretary general said Monday that the alliance is reviewing a permanent increase of NATO peacekeepers to Kosovo and Bosnia at a time of increasing violence and warnings of Russian interference.

In Bosnia and Herzegovina, where Bosnian-Serb leaders have stepped up their rhetoric seeking secession from the federation, NATO's Jens Stoltenberg said that NATO allies 'strongly support the sovereignty and territorial integrity of Bosnia-Herzegovina.'

'We are concerned by the secessionist and divisive rhetoric, as well as malign foreign interference, including Russia,' Stoltenberg said in Sarajevo.

Stoltenberg later on Monday stopped in Kosovo, where on September 24 dozens of heavily armed men led by Kosovo-Serb politician Milan Radojcic - with alleged ties to Serbian President Aleksandar Vucic - attacked Kosovo police in Banjska, killing one officer and engaging state security forces in an hourslong gun battle on the grounds of a Serbian Orthodox monastery.

Three of Radojcic's armed men were killed before the attackers fled to neighboring Serbia, where U.S. national security officials said they monitored "an unprecedented" but brief Serbian military buildup along the border with Kosovo's Serb-dominated north. The Serb military presence along the border, which included artillery, tanks and mechanized infantry units, dissipated after U.S. warnings.

Stoltenberg said NATO allies, who responded to the late-September fighting by deploying an additional 1,000 peacekeepers, found the violence "serious" and added more patrols in the area.

"We are now reviewing whether we should have a more permanent increase to ensure that this does not spiral out of control and create new violent conflict in Kosovo or in the wider region," Stoltenberg said during the joint news conference in Pristina with Kosovar President Vjosa Osmani, which followed meetings with national leaders.

Officials in Pristina have blamed their counterparts in Belgrade for orchestrating the Banjska attack, which officials in Belgrade deny and portray as a reaction by local Serbs disgruntled with Kosovo's government.

FILE - U.S. members of the NATO-led Kosovo Force (KFOR) stand guard outside municipal offices in Leposavic, Kosovo, June 2, 2023.

Eye on Russia

Stoltenberg's visit to Bosnia and Kosovo comes amid increased fears that Russia may use Serbian leaders in Bosnia - and Serbia's rejection of Kosovo's independence - to enflame hostilities in a region where ethnic tensions remain rife.

"We expect that those who are responsible for this absolutely unacceptable violence are held to account," Stoltenberg said. "It is important that it has consequences because we need to do whatever we can to ensure that this kind of violence doesn't happen again."

The September gun battle followed another violent clash in May, when 93 NATO peacekeepers were injured by a Serbian mob that protested a move by Kosovo Prime Minister Albin Kurti to install Albanian mayors in municipal buildings after a local election boycotted by Kosovo Serbs.

Kosovo seceded from Serbia in 2008, when it fought a war of independence following Yugoslavia's disintegration. Serbia's 1998-99 crackdown in Kosovo ended after NATO's bombing campaign and a U.N. resolution that forced Serbia to relinquish control over the territory.

"NATO has a history in this part of Europe since the 1990s and, therefore, we have invested a lot in stability, in peace in this region, in Kosovo but also in Bosnia," Stoltenberg said. "We are really investing in security in this region because safety for these people in this region, stability in this region matters for our own security."

Leaders from Kosovo and Serbia are engaged in a EU-mediated dialogue that aims to normalize relations between the two. The dialogue, however, has largely stalled amid opposing demands between the two sides.

Stoltenberg urged both sides to refocus the dialogue.

"Stability in the region depends on all sides choosing diplomacy over violence and in honoring existing commitments," he said.

Stoltenberg's tour of the region is scheduled to include a stop in North Macedonia.