(Adds Serbian minister, paragraphs 13-14)

By William Schomberg

LJUBLJANA, Jan 8 (Reuters) - New European Union president Slovenia said on Tuesday it wanted Serbia to take a key step towards future membership this month, but acknowledged some EU states needed to be convinced.

Slovenian Foreign Minister Dimitrij Rupel told reporters he wanted Serbia to sign a Stabilisation and Association Agreement, the first rung on the ladder to eventual accession.

"I am one of those who believe the SAA should be signed as soon as possible, possibly by the end of this month, but some of our colleagues have to be persuaded," he said.

Serbia has been stopped from signing the SAA because of its failure to arrest and transfer former Bosnian Serb commander Ratko Mladic to the tribunal in the Hague on genocide charges.

EU enlargement chief Olli Rehn reiterated Belgrade must first cooperate fully with the U.N. war crimes tribunal on the former Yugoslavia.

Serbia must have the unanimous agreement of the 27 EU states before it can sign the SAA.

Rupel acknowledged some capitals had problems, but there is growing pressure in the EU to take the step to help pro-European reformist President Boris Tadic, who is seeking re-election in a vote scheduled for Jan. 20 with a second round likely on Feb. 3.

Former chief war crimes prosecutor Carla del Ponte, whose term ended last month, opposed the EU signing the SAA before Mladic's capture, saying that was the strongest leverage to make Serbia comply.

Rehn said full cooperation with the Hague tribunal "has been, and this is still and will be the essential condition for the signature of the SAA agreement with Serbia".

EU IS JUDGE

However, Rupel said it was the EU that would have the final say on what constituted full cooperation. He and Rehn said they would meet Del Ponte's successor, Belgian Serge Brammertz, soon.

Rupel later said the EU presidency and commission would propose a "Serbia task force" to help speed accession to the bloc, including ways to boost cooperation with the Hague.

The working group, along the lines of a similar group that helped Croatia qualify to start accession talks with the EU, would comprise officials from Slovenia, France, which takes over the EU presidency in July, and other EU officials, he said.

Serbian Foreign Minister Vuk Jeremic said after talks with EU foreign policy chief Javier Solana in Brussels that Belgrade hoped to be able to sign the SAA on Jan. 28.

"Serbia is cooperating fully with the Hague tribunal and therefore as far as we are concerned all conditions have been met," he told reporters.

However, the Netherlands, home to the war crimes court, has said it will agree to the signing only once Mladic, wanted for the 1995 Srebrenica massacre of some 7,000 Bosnian Muslims, is on a plane to the Hague.

Belgium said the EU should take its lead from the prosecutor in determining whether Belgrade was cooperating.

Rehn was less specific, saying it was up to Belgrade to demonstrate full cooperation that should lead to the arrest and transfer of Mladic, implying there could be a signature while Mladic was still at large.

Rupel underlined the desire to boost pro-Western parties in Serbia, saying: "We hope for a continuity. We would like to see the democratic forces of Serbia winning."

He denied there was any intention to offer closer EU ties to compensate Serbia for the looming loss of Kosovo, the breakaway province expected to declare independence in the coming months. (Additional reporting by Emma Thomasson in Amsterdam, Mark John and David Brunnstrom in Brussels and Matthew Robinson in Belgrade; Writing by Paul Taylor; Editing by Janet Lawrence)

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