A US-backed land-swap plan to redraw the borders of Kosovo is facing a growing chorus of criticism inside the republic, in the wider region and internationally.

The Kosovan president, Hashim Thaçi, and his Serbian counterpart, Aleksandar Vučić, have suggested an exchange of territory could be part of a deal that would pave the way for a final settlement between Belgrade and Pristina.

Kosovo broke from Serbia in 1999, and declared its independence in 2008, which Belgrade has never recognised. Vučić and Thaçi have met regularly under the auspices of the EU’s foreign policy chief, Federica Mogherini, to hammer out a deal that could put Kosovo on the path to a seat at the UN and improve EU accession prospects for both countries.

While neither Vučić nor Thaçi has made the detail of their plans public yet, both acknowledged at a discussion forum in Austria last weekend that border changes were under consideration. A swap would probably involve exchanging territory in southern Serbia, predominantly populated by ethnic Albanians, for part of northern Kosovo with a largely ethnic Serbian population.

The US administration recently gave a boost to the plans when the national security adviser, John Bolton, said Washington would not stand in the way if Belgrade and Pristina reached a deal, reversing a long-held US policy that further border changes in the Balkans are undesirable.

According to two sources familiar with Trump administration’s thinking, the new policy is “no red lines but no blank cheques”, meaning the US is willing to look at any solution, including border changes, but will not necessarily endorse it in the end. Some EU officials have hinted they may also be willing to back a deal that involves border changes.

The Serbian president, Aleksandar Vučić, is due to meet Vladimir Putin twice in the coming months. Photograph: Ronald Zak/AP

Vučić will face opposition from many at home who would see a deal as admission that Kosovo is lost forever. There is also strong criticism inside Kosovo, notably from the prime minister, Ramush Haradinaj, who on Friday referred to the discussions as “disastrous ideas”. He tweeted: “The public discussion on the redesign of borders & exchange of territories is an invitation for new tragedies in the Balkans.”

Many regional and international observers have noted the potential for knock-on effects in the region, where some borders remain fragile and open to nationalist challenges.

The German chancellor, Angela Merkel, has come out strongly against the idea, and three former high representatives to Bosnia have written an open letter to Mogherini, urging her not to back any plans involving land swaps.

“We can in short, think of no policy more likely to lead us back to division and conflict in the Balkans than the one which some are apparently now supporting,” wrote Carl Bildt, Paddy Ashdown and Christian Schwarz-Schilling in the letter.

Milorad Dodik, the president of the Serb-dominated autonomous Republika Srpska (RS) entity and a candidate for the national presidency in October elections, has regularly called for RS independence.

“I think the EU likes this plan because it’s ‘simple’,” said Lejla Ramić-Mesihović, the executive director of Foreign Policy Initiative BH, a Sarajevo-based thinktank that has been working on the issue. “But what logic will the international community have against an RS referendum? Is now really the time to give up on multi-ethnic states in Europe?”

An adviser to Dodik said the concerns were exaggerated. “President Dodik is one of the most experienced politicians in the region and he will do nothing, knowing how important it is for Serbia to close the story with Pristina,” said Ana Trišić-Babić, a former deputy foreign minister of Bosnia.

However, there are fears that simply by putting the issue back on the table, it will give fuel to those who wish to reopen questions that had appeared to be long resolved.

In Macedonia, which has a large ethnic Albanian minority and was the scene of inter-ethnic clashes in 2001, politicians are wary of any deal involving land swaps.

“These ideas have been floating around throughout the Balkan wars, and this country has always been very firm in supporting political solutions to political problems that do not require ethnic borders,” said Radmila Šekerinska, Macedonia’s minister of defence. “We need to remind ourselves that the biggest atrocities were committed with that aim, or with that excuse, in mind.”

Another key question is whether Russia can be brought onside for any deal. Vučić is due to meet the Russian president, Vladimir Putin, twice in the coming months, and so far Moscow has given little indication of whether it is ready to drop its long-held opposition to Kosovan independence.

Maria Zakharova, spokeswoman for the Russian foreign ministry, said it was down to Belgrade and Pristina to agree a deal, but pointedly said “the deal has to reflect the interests of the Serb people” if Russia is to back it. She declined to clarify how Russia would assess these interests.