Al-Qaeda’s second-in-command has poured scorn on President Bush’s plan to send thousands more troops to Iraq, boasting that even the whole US army would not be a match for insurgents in Iraq.

In an online video address, Ayman al-Zawahiri, who carries a $25m bounty on head, is shown mocking Mr Bush’s fresh strategy to send 21,000 extra troops to quell sectarian violence, challenging him to send "the entire army" into the conflict-torn nation so that they could be destroyed by his charges.

The video came as the US military said that it had detained more than 600 fighters from Moqtada al-Sadr’s Mehdi Army, the feared and powerful militia which a recent US report described as having replaced al-Qaeda as the largest threat to security in Iraq.

The recording was posted by the US-based SITE Institute, which searches and analyses militant networks and said it had found the video on al-Qaeda’s multimedia arm. In the 15-minute tape - which marks the first reaction from the terror group to the new Iraq strategy - the Egyptian-born al-Zawahiri is seen taunting the US, wearing his trademark white turban and black cloak.

"In his latest speech, Bush said in his ramblings that he would send 20,000 of his soldiers to Iraq. I ask him: why send only 20,000 soldiers? Why don’t you send 50,000 or 100,000?" he said.

"Don’t you know that the dogs of Iraq are impatient to devour the carcasses of your soldiers? On the contrary, you must send your entire army to be annihilated at the hands of the mujahedeen so that the whole world will be rid of your wickedness."

Mr Bush, facing mounting criticism of his handling of Iraq, announced on January 10 that he would send an additional 21,500 troops in a mission to stem the sectarian violence which has been blamed on al-Qaeda and other Sunni insurgents as well as deadly Shia militia.

The President has waged an aggressive public relations campaign over the past week to warn against pulling out of the conflict-torn nation hastily, and is expected to focus on the issue in his annual State of the Union speech tonight.

But US forces have suffered heavy casualties in Iraq this month, with 49 dead including 12 troops killed in a helicopter crash for which the Iraqi branch of al-Qaeda claimed responsibility in an internet statement posted yesterday.

In the tape, al-Zawahiri also mocks the US-led mission to rid Afghanistan of remnants of the extremist Taliban movement.

"Iraq, the country of the caliphate and of jihad, is capable of being a tomb for ten of your armies. It is al-Qaeda and the Taliban, led by the emir of the believers, Mullah Mohammed Omar…who have deprived the Americans of a safe haven in Afghanistan.

"Security must be shared: if we are safe, you will be too... If we are hit and killed, you will inescapably be hit and killed. Today, the duty of every Muslim is to bear arms or to support and serve those who bear the arms."

Zawahiri also refers to recent events in Somalia in his message, accusing the US of being behind the deployment of Ethiopian troops and vowing that Islamist forces would "break the back" of the Ethiopian forces.

Unlike his leader, Osama bin Laden, Zawahiri frequently speaks for Al-Qaeda in video or audiotapes although his whereabouts remain unknown. He is listed by the US government for being wanted in connections with the 1998 bombings of the US embassies in Kenya and Tanzania, and was sentenced to death in absentia by an Egyptian court a year later.

In a previous online message on January 5, Zawahiri called on the Somali Islamists to be use violence in Iraq and Afghanistan as inspiration to fight the Ethiopian forces in Somalia. The Pentagon said last week that US operations in Somalia would continue until key Al-Qaeda targets were eliminated.

The video came as the US forces said that some 600 fighters and 16 leaders of the Mehdi Army – the militia loyal to radical Shia cleric al-Sadr - had been taken into custody.

A military statement said that in the past 45 days, 52 raids had targeted al-Sadr’s militia, but did not reveal when the members who are awaiting prosecution were detained. It added that a further 42 raids had focused on Sunni insurgents.

The Mehdi Army, which is believed to have up to 60,000 members, forms one of Iraq’s most feared and powerful militias and is suspected of being heavily involved in the killing of Sunnis in Baghdad and other parts of the country. The latest quarterly Pentagon report released last month named it as the largest threat to security, saying it "has replaced al-Qaeda in Iraq as the most dangerous accelerant of potentially self-sustaining sectarian violence in Iraq."

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