The United Nations chief war crimes prosecutor, Carla Del Ponte, has for the first time explicitly accused the authorities in Belgrade of sheltering Ratko Mladic, the wartime military commander who has been indicted for genocide.

In a speech yesterday that sharply increased the political pressure on Belgrade, Ms Del Ponte said that Ratko Mladic is living in Yugoslavia under the protection of the Yugoslav army and is being shielded from justice with the federal government's consent. Ms Del Ponte also attacked the authorities in Republika Srpska, the Serb-controlled half of Bosnia, for harbouring the former Bosnian Serb leader, Radovan Karadzic, who remains at liberty more than six years after his indictment for genocide in the 1992-95 Bosnian war.

In July, the Serbian government of Zoran Djindjic handed over the former Yugoslav president, Slobodan Milosevic, to the war crimes tribunal in The Hague, where he is awaiting trial. That move implied that the net was closing on Mr Mladic, who was assumed to be living in Belgrade and has been seen there at a football match. But co-operation with the tribunal has not been so forthcoming from the federal authorities of the President, Vojislav Kostunica, including the army.

Ms Del Ponte said yesterday: "Ratko Mladic is residing in the Federal Republic of Yugoslavia under the official protection of the Yugoslav Army. As an officer of the Yugoslav Army, General Mladic is said to enjoy military immunity, and he is being shielded from both national and international justice."

That, she said, was an "affront to the authority of this council and mocks the entire process of international criminal justice".

The war crimes tribunal's problems seem rooted in internal political rivalries between Mr Kostunica, who opposed the handover of Mr Milosevic, and the Prime Minister of Serbia, who agreed to it.

The chief prosecutor said that, "working with Prime Minister Djindjic and the Serbian authorities at the republic level, we have experienced good results" and she hailed the surrender of Mr Milosevic as a "groundbreaking event and a courageous step by the Serbian government". However, she added that "co-operation at the federal level appears to be blocked for reasons of domestic politics", and claimed that the federal institutions "obstruct" the tribunal's work.

Officials in the chief prosecutors' office said that the statement was made on the basis of new evidence that Mr Mladic is within Yugoslav borders and has an address in the capital. Florence Hartmann, a spokeswoman for Ms Del Ponte, said: "With Mladic it is a question of political will on the part of a state which is a member of the UN. With Karadzic, too, the authorities know his whereabouts. It is not a big territory and we have no clear explanation of why they were not arrested during so many years."

Yugoslavia's Interior Minister, Zoran Zivkovic, rejected the claim, saying that he had "no knowledge that he [Mladic] is in our territory".

However, Ms Hartmann said that a statement from Mr Kostunica or the minister responsible for the armed forces would be more appropriate.

* The Macedonian President, Boris Trajkovski, said yesterday he is prepared to give "strong consideration" to the extension of Nato's mission in Macedonia and perhaps even a change in the mandate of the 1,000-man force.