An audio recording attributed to Osama bin Laden offers rewards in gold for the killing of top US and UN officials in Iraq or citizens of any nation fighting there, report agencies. 

The 20-minute recording, which appeared on at least two websites known for militant Islamic messages, offers 10 kilograms of gold for the killing of chief administrator in Iraq Paul Bremer, UN Secretary-General Kofi Annan or UN special envoy to Iraq Lakhdar Brahimi, and smaller amounts for other Western officials in Iraq.

 The authenticity of the recording could not immediately be verified. The is the first time that Bin Laden has offered rewards for missions which he considers to be a religious duty, and the CIA is trying to verify the message's authenticity, the BBC said.

"You know that America promised big rewards for those who kill mujahedeen (holy warriors)," the speaker said. "We in al-Qaeda organisation will guarantee, God willing, 10,000 grams of gold to whoever kills the occupier Bremer, or the American chief commander or his deputy in Iraq."

The US price on bin Laden's head now stands at $US50 million.  

 "For security reasons, the rewards will be given as soon as conditions permit, God willing," the voice said. "As for those who die while killing an occupying soldier, the great prize will be for us and for him when God grants him martyrdom, and the smaller prize (the gold) will be for his family."

 "The United Nations is nothing but a Zionists' tool, even if it worked under the cover of providing humanitarian aid," the voice said. "... Whoever kills Kofi Annan or the head of his commission in Iraq or a representative like Lakhdar Brahimi, he will be awarded the same prize of 10,000 grams of gold."

The speaker also promised a kilogram of  gold - to anyone killing a citizen of  "the masters of the veto like Americans and Britons", said to be a reference to nations with veto power on the UN Security Council.