Bulgaria extends arrest of ex-Kosovo PM

* Ceku wanted in Serbia on war crime charges

* Bulgaria to decide on extradition

(Adds Serbia seeking extradition, paragraphs 5-6)

SOFIA, June 24 (Reuters) - Prosecutors said on Wednesday former Kosovo Prime Minister Agim Ceku, wanted in Serbia on war crime charges, would stay in custody in Bulgaria for 72 hours while Sofia decided whether to extradite or release him.

Bulgarian police arrested Ceku at the border with Macedonia on Tuesday night on an old Interpol arrest warrant issued by Serbia.

Sofia was awaiting documents from Interpol and would decide how to proceed by Friday, Bulgaria's Chief Prosecutor Boris Velchev told reporters. His office later said a decision was expected later on Wednesday.

Serbia indicted Ceku, a former top commander of Kosovo Albanian fighters during the 1998-1999 war, and some other Kosovo leaders, including current Prime Minister Hashim Thaci, on suspicion of committing crimes against Serbs.

Serbia's justice ministry said on Wednesday it filed a request to Sofia for extradition of Ceku, Tanjug news agency reported.

"I hope that Bulgaria will promptly act upon our request and that the answer will be positive," Justice Minister Snezana Malovic was quoted as saying by Tanjug.

The Kosovo government has asked Interpol and other countries to ignore Serbia's request.

Ceku was invited to Bulgaria by former foreign minister Solomon Passy. Passy told local media Ceku was involved in an initiative helping Kosovo move closer to joining NATO.

This is the fourth time Ceku has been arrested, but on earlier occasions authorities in Slovenia and Hungary did not hand him over to Belgrade.

Last month, Ceku left Colombia following an expulsion order from the local security agency after Serbian authorities asked their Colombian counterparts to arrest him.

Kosovo's Albanian majority, backed by Western countries, declared independence in 2008 and was recognised by 60 countries including Bulgaria. Serbia has said it will never recognise an independent Kosovo. (Reporting by Anna Mudeva and Ivana Sekularac in Belgrade; Editing by Charles Dick)