The European Commission's roadmap for the region unveiled earlier this month said the countries must root out problems with corruption and the rule of law, and, especially, settle a series of simmering territorial disputes.
"The EU door is open to further accessions when, and only when, the individual countries have met the criteria," the plan said.
European Commission head Jean-Claude Juncker will start a tour of the region on Sunday to discuss the new strategy, which stresses "implementing fundamental reforms and good neighbourly relations".
Montenegro and Serbia are the frontrunners to join, with Albania, Bosnia, Kosovo and Macedonia lagging behind, but all are getting impatient after the EU put expansion on hold four years ago.
The EU has been wary of admitting new members before they settle their differences, and the border rows will be a particular point of contention in a region still bedevilled by the aftermath of the bloody break-up of Yugoslavia in the 1990s.
Serbian President Aleksandar Vucic said the chance to join by 2025 was "very important as an incentive for Serbian citizens".
But he warned that it would take some "difficult decisions" to solve the country's outstanding issues -- first and foremost, Serbia's dispute with Kosovo.
Serbia has refused to recognise its former breakaway province since it declared independence a decade ago.
'Ball in our court'
Montenegro has reason to be satisfied with the EU's strategy, according to Prime Minister Dusko Markovic, but the European Movement Montenegro found it was "discouraging" that the plan considered 2025 to be an "extremely ambitious" prospect.
Juncker has warned Serbia and Montenegro against too much "excitement", saying that 2025 "is an indicative date, an encouragement date so that the people concerned can consistently set out on their way".
The deadline for the other Balkan nations is less clear.
Albania, a candidate since 2014, hopes to open talks with Brussels this year and is willing to speed up reforms to catch up with the others, according to Prime Minister Edi Rama.
"We want to make progress faster and, why not, join EU at the same time" as Serbia, Rama said.
But Albanian political analyst Lutfi Dervishi warned that joining the EU "is a national desire but it is not a lottery".
"The ball is in our court," Dervishi told AFP, referring to the pace of reform, particularly in the fight against organised crime and corruption, seen by the EU as a priority for Albania.
'Locked in the waiting room'
Macedonia, an EU candidate since 2005, will have to resolve a festering 25-year row with Greece over its name.
Athens, which can block Macedonia's EU membership, argues its neighbour's use of the name suggests Skopje has territorial claims to Greece's historic northern region.
But Macedonia has made significant progress recently and the EU said it is "very confident" the dispute will be settled before July.
Prime Minister Zoran Zaev has praised the EU's new plan, saying that "according to this strategy Macedonia is on the right track".
He added it would provide motivation "to work and catch up with Serbia and Montenegro, who already negotiate" with the EU.
But Macedonia's Foreign Minister Nikola Dimitrov said earlier this month that it was difficult for countries to keep moving in the right direction if they are "locked in the waiting room".
In Bosnia, which applied for membership in 2016, Muslim leader Bakir Izetbegovic said he was confident "we can catch up with those currently ahead us on the path towards the EU".
But the country will have to tackle ongoing tension with Bosnian Serbs, including their leader Milorad Dodik, who regularly threatens secession.
Kosovo was the most disappointed with the EU's strategy.
Media in the capital Pristina pointed out that rather than employing terms like "accession" and "candidate status" used for other countries, the document said Kosovo could "advance on its European path" once "circumstances allow".
Five of the EU's 28 member states still do not recognise Kosovo's independence, and President Hashim Thaci has blamed them "for the lack of a unique stand for Kosovo" in the new plan.
"But Kosovo will not sink into despair," Thaci said.
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