US aircraft attacked Shia militia in Basra for the first time in the current round of fighting as intense battles continued between supporters of Moqtada al-Sadr and tens of thousands of Iraqi forces in a crackdown personally supervised by Iraq's prime minister, Nouri al-Maliki.
British troops, based at the city's airport, were kept away from the operation described by George Bush as "a defining moment in the history of Iraq".
American fighter jets dropped bombs on a mortar team and a militia stronghold in Basra, said Major Tom Holloway, a British military spokesman. The number of casualties was unknown.
As protests spread across Iraq, US aircraft also attacked Sadr City in eastern Baghdad, killing at least five civilians, according to Iraqi police and hospitals.
"There have been engagements going on in and around Sadr City. We've engaged the enemy with artillery, we've engaged the enemy with aircraft, we've engaged the enemy with direct fire," Major Mark Cheadle, a US military spokesman in Baghdad, said later.
Defying a curfew, protesters again attacked the US-protected Green Zone in the capital with mortars and rockets. Elsewhere at least 22 people, including six civilians, were killed in fierce fighting in the southern cities of Mahmoudiya, Nasiriya - now held by elements of Sadr's Mahdi army - and Kut, according to reports from police and army officials cited by news agencies.
In Basra, Abdel Qader Jassim, the defence minister, admitted Iraqi security forces were "surprised" by the resistance they had met. At least 120 people, described by Iraqi commanders as the "enemy", were reported to have been killed and 450 wounded in four days of fighting.
Maliki, head of the Shia-dominated government, initially set a 72-hour deadline for fighters to give up their weapons. He extended that to April 8 yesterday and offered cash for any heavy weapons handed over in a clear indication that the offensive by 15,000 Iraqi troops and a similar number of police, would take longer than planned.
British defence officials said they were carefully monitoring a "developing operation likely to take time". British forces were providing medical and logistical support and air surveillance, they said.
But they made it clear that the British government and military commanders did not want to intervene. "The operation was planned, implemented, and executed by the Iraqis. We will only intervene if requested by the Iraqis," the MoD said.
As if to drive home the point, an official added: "It is their operation, their responsibility to bring security to Basra and Iraq as a whole."
Gordon Brown's spokesman said: "The fact that the Iraqi security forces are able to engage in this way in Basra is a recognition of the training and support that they have been given in recent years, that they are now able to take on a much greater degree of responsibility for their own security."
Bush praised Maliki on Thursday for his "bold decision" to strike at the militia in the country's oil-rich and port region but was careful not to urge Britain to intervene on the ground.
There are 4,100 British troops at the airport base. Brown suggested at the end of last year that the number could be reduced to 2,500 this spring.
The increase in violence means such a cut is unlikely. But it also raises the question of what is the point of having any there, a point not lost on defence chiefs and ministers in London. They are desperate for a convincing success by Iraqi forces even if it takes a few days more than first hoped.
There was little sign of that yesterday. Television showed masked fighters moving around freely in a south-western neighbourhood of Basra but little traffic.
Basra's citizens were reported to be complaining they were being deprived of food, medicines, electricity and water.
Maliki has said that the operation, nicknamed Sawlat al-Fursan or Charge of the Knights, is targeting criminals, not Sadr's militia, and has vowed "no retreat". But distinguishing the Mahdi army from criminals appears to be cutting little ice among Sadr's supporters, many of whom believe that US and Iraqi forces have used a seven-month ceasefire to prepare to attack them. Many Sadrists believe the Baghdad government is siding with the Badr organisation, a rival militia allied to the Supreme Islamic Iraqi Council led by Abdul Aziz al-Hakim.
Sadr called for a political solution to the crisis on Thursday and an end to the "shedding of Iraqi blood". But the statement stopped short of ordering the Mahdi army to stop attacks. One of his representatives called Maliki "a hypocrite" during a Friday sermon yesterday in which he also called for an end to military operations, Reuters reported.
In Baghdad, the Iraqi parliament called an emergency meeting but only 54 members of the 275-seat body succeeded in getting inside the fortified Green Zone.