Belgrade - Serbian and Kosovar presidents Aleksandar Vucic and Hashim Thaci are set to resume talks in Brussels after a summer that broke the taboo of possible border changes between the former war foes.

The two leaders will meet on Friday but critics say the proposal could have a dangerous domino effect in a fragile region scarred by the bloody break-up of Yugoslavia.

What is the problem?

The possible land-swap is being floated as a solution to Serbia and Kosovo's "frozen conflict."

The heart of the dispute is Serbia's refusal to recognise Kosovo's independence, which the former southern province declared in 2008.

Belgrade effectively relinquished control over the majority-Albanian region after 1999 NATO bombings forced the end of a bloody war between Serb troops and Kosovar separatists.

Yet Serbia still regards Kosovo, now recognised by more than 110 countries, as a rebel province and has blocked its entry into the UN with help from Russia.

In a bid to normalise relations -- a prerequisite for Serbia joining the EU -- Pristina and Belgrade started a dialogue in 2011.

The talks have stalled for months but started generating buzz after Vucic and Thaci signalled an openness to the idea of border changes this summer.

Map showing Balkan states' status regarding the European Union

Which borders could change?

The presidents have not laid out any detailed plans, instead using vague language like "border adjustments" and "demarcation."

But Thaci, for his part, has spoken publicly of wanting to bring Presevo Valley -- an Albanian-majority strip of southern Serbia -- into Kosovo.

Presevo Valley is home to up to 60,000 ethnic Albanians, as well as around 20,000 Serbs.

The speculation in local media is this valley could be traded for northern Kosovo, a Serb-dominated region that Vucic will visit on Sunday.

Belgrade has kept strong links with the 120,000-strong Serb community in Kosovo which also refuses to recognise Pristina's authority.

Kosovo is now recognised by more than 110 countries

What are the risks?

Critics have been sounding the alarm that a land-swap would open a "Pandora's box" of conflicts in the region.

Ethnic hatreds fuelled the bloody dissolution of Yugoslavia in the 1990s, wars that left 130,000 dead and displaced millions as borders were redrawn.

Decades later, relations between ethnic groups remain precarious in many mixed states, such as Macedonia and Bosnia.

There is particular concern about a knock-on effect in the Serb-run entity of Bosnia, whose president has repeatedly called for independence.

Those fears were captured in a poster erected in a small town of the entity this summer of Ratko Mladic, a Serb commander convicted of leading the Srebrenica genocide of Bosnian Muslims in 1995.

The poster, placed by Serb veterans in Bosnia who regard Mladic as a hero, bore one of his quotes: "It is with blood that borders are drawn".

"The illusion that ethnically 'pure' entities and nation states would be a solution are not new, their architects have already been condemned in The Hague," warned Lejla Ramic Mesihovic, from the Sarajevo-based think tank Foreign Policy Initiative BH.

What does the West think?

For some observers, the most worrying dimension is an apparent change of heart among some Western powers, who once opposed redrawing the map in the Balkans.

US national security advisor John Bolton said last month that the States would not "exclude territorial adjustments" from a possible accord.

Senior EU officials, including Enlargement Commissioner Johannes Hahn, have also refused to rule out border changes.

The only vocal critics have been the UK, Austria and Germany.

"We believe that this can tear open too many old wounds in the population. And therefore we are very sceptical at this point," Germany's Foreign Minister Heiko Maas said last week.

EU foreign policy chief Federica Mogherini, who is leading the Kosovo-Serbia talks, has refused to comment on the specifics.

Will this really happen?

There is scepticism that a land-swap could take place soon, if ever.

Thaci must contend with criticism from an opposition at home, plus a prime minister who has repeatedly condemned the idea, saying it would "mean war".

Vucic, meanwhile, has been taking heat from the powerful Serbian Orthodox Church, which regards Kosovo as the "Jerusalem of the Serbs."

Its leader Patriarch Irinej has warned that "what is taken by force can be taken back, while what is given is lost forever".

Some analysts speculate the presidents are ratcheting up tension with plans to work down to a more palatable resolution.

By Nicolas Gaudichet and Sally Mairs