By Roland Watson in Washington and Philip Webster

BRITISH and American forces were shelling targets in Basra last night to support an apparent uprising against the militias controlling Iraq’s second city.

An American warplane was said to have destroyed the headquarters of the ruling Baath party with a 2,000lb bomb, and British troops pummelled the city with rockets and shells.

The 7th Armoured Brigade Desert Rats intervened after military intelligence reported that Fedayin security forces were firing on civilians demonstrating outside their headquarters. The scale of the protest was not immediately clear, but there were reports of thousands of rebels and of buildings ablaze.

British troops had struck at Basra earlier in the day, capturing a top Baath party official. But they also suffered further casualties during fighting outside the city: two men were killed and two wounded when one British Challenger II tank fired a single shell at another.

Tanks of the Royal Scots Dragoon Guards were later reported to be shelling units loyal to President Saddam Hussein that were firing mortars on the rebels, and American jets joined the attack.

Iraqi ministers denied that there was any uprising and British military chiefs admitted that details of the protests were sketchy. Major-General Peter Wall, Britain’s second-in-command, said that if there were a revolt that would be encouraging and every attempt would be made to capitalise on it. Allied forces would assist “wherever we can”.

Washington has done its utmost to foment rebellion in Basra in its quest to portray the war as one of liberation rather than occupation. The city’s 1.3 million people are predominantly Shia Muslims and they have been bombarded with leaflets encouraging them to rise up against Saddam’s Sunni regime as they did after the 1991 Gulf War.

Then the allies failed to go to their aid and the revolt was crushed. Yesterday Tony Blair acknowledged that failure and promised: “This time we will not let you down. Saddam and his regime will be removed.”

But Donald Rumsfeld, the US Defence Secretary, was wary of encouraging people to revolt. “We know there are people ready to shoot them if they try to rise up, we know there are people who will kill them if they try to leave,” he said. “These are an oppressed people. I hope and pray they’ll do it (rebel) at a time when there are sufficient forces near by to be helpful to them.”

News of the apparent uprising came hours after President Bush and Mr Blair had sought to stiffen allied resolve after further setbacks. Blinding desert sandstorms had slowed the coalition advance on Baghdad to a crawl and two American helicopters went missing.

Apache helicopter gunships operating against Republican Guard units south of Baghdad were forced down in mid-mission, and advances by other units were halted as visibility was cut to five yards.

Speaking before the “friendly fire” tank crew deaths, Mr Blair said: “Of course there will be accidents and tragedies. War is always like that. But the strategy is taking shape exactly as we thought it would.”

Mr Bush, who will hold a council-of-war with Mr Blair at Camp David today, conceded that more reverses were possible, but insisted that the final outcome was not in question. “We cannot know the duration of this war, yet we know its outcome. We will prevail.”

The bombing of Baghdad continued throughout the night, apparently as a “softening up” exercise before the arrival of the allied forces, and 25 explosions were heard on the southern outskirts as dawn broke. There had earlier been huge blasts in the city centre near the Ministry of Information and state television tower. Iraqi television was blacked out for 45 minutes before resuming broadcasts.

In Najaf, the US 7th Cavalry was involved in the heaviest battle of the campaign and the Pentagon reported that 300 Iraqi troops had been killed.

Pentagon chiefs said there may be weeks of tough fighting before Baghdad is secured and that the battles could become more dangerous. Fears that Iraqi forces may use chemical weapons grew yesterday after the discovery of more than 3,000 chemical suits with masks, and stocks of the nerve gas antidote atropine, at a hospital in Nasiriyah.

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