International Criminal Court judges on Wednesday pressed Libya to immediately hand over slain leader Moamer Kadhafi's son Seif al-Islam to stand trail for crimes against humanity.

The Hague-based ICC "reiterates its request that Libya make its decision to grant the surrender request and proceed immediately with the surrender of (Seif al-Islam) to the court," it said in a public decision.

It was the second time judges asked Tripoli to hand over Seif, 39, after last month rejecting an initial Libyan request for a postponement in order to put him on trial in the north African country itself.

Libya said it would challenge the ICC decision, but so far no challenge has been filed before the court, ICC spokesman Fadi el-Abdallah told AFP.

The ICC said Wednesday: "Absent any justification for postponing the execution of the surrender request the government of Libya must therefore: make a decision to grant the surrender request".

It added Tripoli should start making arrangements for Seif's surrender to the court "without further ado".

Should Libya fail to comply it "may warrant that the court make a finding to that effect", but did not elaborate.

Seif, seen as Kadhafi's de facto prime minister, was arrested on November 19, and remained in the custody of the military council of Zintan, a town 180 kilometres (110 miles) southwest of Tripoli, a rights organisation said Wednesday.

He, together with former Libyan spy master Abdullah al-Senussi, 62, who was picked up in Mauritania last month, are wanted by the ICC on charges of crimes against humanity committed in trying to put down Libya's bloody uprising which was sparked in mid-February last year.

Human Rights Watch said Seif remained in custody in Zintan and it was unclear whether he has seen a lawyer.