Mr Robertson also said an airfield south-west of Baghdad had been targeted, where Sadam Hussein was developing remotely-operated aircraft with which to deliver chemical and biological weapons against his neighbours.

One aerosol-load of chemical agent delivered by such an aircraft would kill everyone within a five-mile downwind range, he said.

"Sadam's ability to make and use chemical and nerve weapons is being reduced with every day and with every raid on his military structures," Mr Robertson added.

The raids, along with the US, would continue until such times as Sadam Hussein's military machine had been "sufficiently damaged".

BBC Middle East Correspondent Jeremy Bowen reports

The Iraqi authorities have said at least 25 people were killed in Baghdad and 75 wounded although there has been no independent confirmation of the casualty figures.

There was an intense bombardment of targets close to the city centre, where ministries and other government departments are concentrated.

In Baghdad, at least one building was set on fire after powerful blasts lit up the sky in a series of attacks. One hospital suffered blast damage, with many windows blown out.

US sources have indicated the attacks on the Iraqi capital are only part of a much wider operation that takes in targets throughout the country.

However, the Pentagon is still cautious about giving too many details about the operation.

The Pentagon said B-52 bombers launched more cruise missile strikes, while Britain said its Tornado aircraft were involved in the aerial bombardment for the first time.

The diplomatic repercussions of the military action widened late on Thursday when Russia decided to withdraw its ambassador from the United States, Yuli Vorontsov, in protest.

UK Defence Secretary George Robertson: British pilots played a significant part

Later, Russia announced the recall of its ambassador to London, Yuri Fokin.

The UK Prime Minister, Tony Blair, and US President Bill Clinton have again been explaining the decision to attack Iraq.

Mr Blair told the House of Commons in London there was with no alternative other than military action after the Iraqi President, Saddam Hussein, refused to co-operate with United Nations weapons inspectors.

Avoiding civilian casualties

The US president said every effort was being made to avoid hitting civilian targets although he admitted there would be unintended casualties.

President Clinton defends the air-strikes - and their timing

Mr Clinton dismissed allegations that the strikes were timed to divert attention in Washington away from the looming impeachment vote against him.

US officials - and their UK counterparts - have warned that the onslaught will go on for as long as is required.

And Vice-President Al Gore hinted the US may be prepared to continue bombing through the Muslim holy month of Ramadan, which starts at the weekend.

Mr Gore said: "We have always been sensitive to the holy days of Ramadan ... but we are not going to specify when this military action will conclude."

Russian anger

In Russia, there was unanimous anger. President Boris Yeltsin said the strikes violated the United Nations charter. Russia is also threatening to delay signing the Star II nuclear disarmament treaty.

Some are suggesting a cynical motive behind the bombings

Russian Prime Minister Primakov repeated calls for the chief United Nations weapons inspector, Richard Butler, to be sacked.

Mr Butler has been criticised for his damning report on Iraq's efforts to impede the inspection teams. Iraq accuses him of acting in the interests of the US.

Mr Butler said: "It was based on the experts of Unscom. It danced to no-one's tune.

"It was not written to anyone's purposes. It was my report, as promised, on time and based on the facts."

Jeremy Bowen: Extremely powerful missiles are being launched against Baghdad

China, India and Pakistan have criticised the attacks. The Arab League denounced the action as an act of aggression, along with Syria, Lebanon and the Palestinians.

But several countries have made a point of supporting the attacks, among them Japan, and a number of EU members.

Security targets

On Wednesday, US forces hit more than 50 separate Iraqi targets, according to the Chairman of the US Joint Chiefs of Staff.

The Pentagon showed photos of damage to Iraqi targets

General Henry Shelton said more than 200 Tomahawk cruise missiles were fired and 70 US Navy planes were also involved.

He said the attacks had been aimed at Iraqi intelligence operations, and the headquarters of special forces.

Iraqi Foreign Minister Mohammed Said al-Sahhaf said the headquarters of the Iraqi security police and the military intelligence had been hit - as had the house of President Saddam Hussein's youngest daughter, Hala.

Click here for a summary of the military build-up

US Defence Secretary William Cohen said: "We are satisfied that we got the right targets.

Mr Cohen said the US expected retaliatory attacks on American interests around the world, and had taken precautions.