Serbia's failure to hand over fugitive general Ratko Mladic has jeopardised talks with the EU scheduled for April. The BBC's Oana Lungescu in Brussels looks at the EU's "carrot and stick" approach in the Balkans.
The enlargement commissioner Olli Rehn likes to talk about the EU's "soft power." It is a concept that the Harvard academic Joseph Nye defined as "the ability to get what you want through attraction rather than coercion."
An ultimatum over Gen Mladic might bolster Serb nationalists
The magnetic attraction of EU membership has indeed acted as a powerful force for change in the countries of the former communist bloc, most of which joined the EU in 2004.
From that point of view, enlargement has proved the EU's most successful policy or, put bluntly, its biggest carrot.
The promise of EU membership has also helped hold the Balkans together, most clearly in the Former Yugoslav Republic of Macedonia.
In 2001, the signature of a stabilisation and association agreement - similar to that being negotiated by Serbia - was one of the factors pulling the country back from the brink of civil war between the security forces and ethnic Albanian rebels.
Under EU and Nato pressure, changes were gradually introduced granting ethnic Albanians more rights. So last December, in recognition of its efforts to build a multi-ethnic state, Macedonia was given the status of an official EU candidate.
Wielding the stick
But carrots are not always enough. The EU has increasingly shown it is ready to wield the stick too - or "conditionality," as they call it in Brussels.
Bulgaria and Romania have been told they must shape up or join the EU a year later than they expect.
For Turkey and the Balkans, the penalty is even tougher. Negotiations on closer ties can be suspended at any time if they fail to keep their promises.
Speaking during a tour of the Balkans earlier this month, Mr Rehn insisted the EU would remain firm in demanding that aspiring members fulfil all the requirements before they join. Conditionality brings concrete results, he said, as "demonstrated by the fact that a certain general Gotovina is behind bars while a certain author Pamuk is not".
The Turkish government has just dropped a case against Orhan Pamuk, the country's best-known novelist, for remarks about the massacre of Ottoman Armenians, after warnings that its bid for EU membership could be endangered.
Late last year, the Croatian general Ante Gotovina, the third most-wanted man on the list of the UN War Crimes Tribunal for the former Yugoslavia, was finally arrested after the EU took an unprecedented step, by putting off opening planned membership talks with Zagreb.
Even when it resorts to sanctions, the EU does so gradually and with care. The view in Brussels is that this should not be seen as an ultimatum to Serbia.
We want to send a stronger message than before, said a seasoned EU diplomat, but experience shows that when you crack the whip too violently, you usually get the opposite reaction to what you want.
Croatian general Ante Gotovina was arrested after heavy EU pressure
We have to keep on the pressure, but we have to be very cautious about how we do it, adds an EU official.
An ultimatum risks humiliating the Serbs and playing into the hands of the nationalist opposition at a highly sensitive time.
Talks on the final status of Kosovo started last week, and the province - nominally part of Serbia but an international protectorate since 1999 - could gain some form of independence by the end of the year, while Montenegro is planning a referendum on independence this spring which could end its loose federation with Belgrade.
For all these reasons, Mr Rehn may not recommend a formal suspension of negotiations with Serbia. The word he has used repeatedly is "disruption."
This could include a whole range of options with the same result: that talks will simply not continue as if nothing has happened. The warning will be immediately communicated to the Serb Foreign Minister Vuk Draskovic, in Brussels for long-scheduled talks with his EU counterparts.
However, as in the case of Croatia's postponement, the EU is unlikely to set a firm deadline. To the outside world it may look like a fudge, in Brussels it is called "constructive ambiguity".
Mr Rehn told the BBC that Serbia had a "continuous deadline". He made clear that the next round of talks on the association agreement, scheduled for 5 April in Belgrade, would certainly be affected, but that decisions could be taken earlier if needed.
Mr Rehn's hope is that by then, "a certain general Mladic" may be behind bars too. That would open the way for Serbia to sign the stabilisation and association accord by the end of the year and look forward with greater confidence to a European future.