The international community’s chief representative in Bosnia has warned that the country is in imminent danger of breaking apart, and there is a “very real” prospect of a return to conflict.

In a report to the UN seen by the Guardian, Christian Schmidt, the high representative for Bosnia and Herzegovina, said that if Serb separatists carry out their threat to recreate their own army, splitting the national armed forces in two, more international peacekeepers would have to be sent back in to stop the slide towards a new war.

International peacekeeping duties in Bosnia are currently the task of a residual EU force (Eufor) that is 700 strong. Nato retains a formal toehold with a headquarters in Sarajevo. The year-long mandate for both is up for renewal this week at the UN security council, but Russia has threatened to block a resolution unless all references to the high representative are removed, potentially undermining Schmidt’s authority as the overseer of the 1995 Dayton peace deal.

In his first report since taking up the post in August, Schmidt, a former German government minister, warned that Bosnia was facing “the greatest existential threat of the postwar period”.

The Bosnian Serb leader, Milorad Dodik, is threatening to pull out of state-level institutions, including the national army built up with international assistance over the past quarter century, and reconstitute a Serb force. On 14 October, Dodik said he would force the Bosnian army to withdraw from the Republika Srpska (the Serb half of Bosnia) by surrounding its barracks and that if the west tried to intervene militarily, he had “friends” who had promised to support the Serb cause, a presumed reference to Serbia and Russia.

Bosnian Serb police carried out “counter-terrorist” exercises last month on Mount Jahorina, from where Serb forces bombarded Sarajevo throughout a 1992-95 siege.

“This is tantamount to secession without proclaiming it,” Schmidt wrote in a report delivered to the UN secretary general, Antonio Guterres, on Friday. He said Dodik’s actions “endanger not only the peace and stability of the country and the region, but – if unanswered by the international community – could lead to the undoing of the [Dayton peace] agreement itself.”

The high representative said it was possible there would be clashes between Bosnian national law enforcement agencies and Bosnian Serb police.

“Should the armed force of BiH [Bosnia and Herzegovina] splinter into two or more armies, the level of international military presence would require reassessment,” Schmidt warned.

“A lack of response to the current situation would endanger the [Dayton agreement], while instability in BiH would have wider regional implications,” he said. “The prospects for further division and conflict are very real.”

Schmidt’s warnings were delivered as the UN security council was preparing its annual resolution renewing the peacekeeping mandate for Eufor and the Nato headquarters, with a vote as early as Wednesday. Moscow is threatening to block the resolution unless all references to the high representative are removed.

The Kremlin opposed Schmidt’s appointment by a Peace Implication Council, an ad hoc multinational body set up to implement the Dayton peace agreement, and refuses to recognise his authority.

“I suspect what Russia really wants is to chip away at the authority of the high representative’s office by stopping him briefing the council,” said a diplomat close to the discussions.

Kurt Bassuener, co-founder and senior associate of the Democratization Policy Council, a Berlin-based thinktank, said: “It sounds like the Americans, the Brits and the French have effectively agreed to really strip back the references to the high representative that were boilerplate, standard issue language in all the previous resolutions.” He added: “And while legally that doesn’t undercut the high representative, politically it sure as hell does.”

Even if Eufor’s mandate is renewed, there is little appetite in the EU to beef up the small force left in Bosnia. Some member states, particularly Hungary, are supportive of Dodik.

“I think he’s willing to gamble on the possibility that, as improbable as it is, he can get away with it by essentially creating new facts on the ground rapidly and counting on the idea that confusion and delay would grip both Sarajevo and the international community and there would ultimately be no meaningful international consequences,” Jasmin Mujanovic, a Bosnian political scientist, said.

US deputy assistant secretary of state Gabriel Escobar told Congress last week that the US is working with the EU to “make sure there are consequences for any illegal or any destabilising actions” in Bosnia. But it is unclear whether the Biden administration would support a return to Nato peacekeeping.

Alida Vracic, the head of a Bosnia-based thinktank, Populari, said the perpetual and worsening sense of crisis allows the country’s leaders to disguise their failure to govern.

“Dodik has gone ballistic, but collectively politicians hope to win points on this crisis and citizens are the only losers as expected,” Vracic said.

“This discussion comes in handy, as it derails all meaningful discussions like the fact that Sarajevo is choking in smog, that [the regional governments] embezzled money in the Covid-19 crisis, that the death toll in Sarajevo is worse than during the war, the fact that half of the country is living in poverty, that we have completed exactly zero reforms, that no laws have been discussed in the parliament for months.”