Mehdi Army fighters in Basra have defied government calls to disarm
British forces have become directly involved in the fighting in Basra, as clashes continue between the Iraqi army and militiamen of the Mehdi Army.
The UK military said troops had launched artillery shells at a mortar crew in the al-Khalaf area of northern Basra, which had fired on Iraqi troops.
It is the first time they have directly joined the fighting since the Iraqi army operation began on Tuesday.
The British Army spokesman in Basra said the engagement had been a success.
Iraqi government forces have been trying to wrest control of Basra and other Shia areas from the Mehdi Army - a Shia militia loyal to the radical cleric Moqtada Sadr.
Across Iraq, fighting has claimed an estimated 200 lives since Tuesday.
The Iraqi Prime Minister, Nouri Maliki, has given the Mehdi Army until 8 April to lay down their arms - which they have refused to do.
In Basra, Britain has been playing a back-up role in the operations. British aircraft have been patrolling the skies, but have not been used to attack.
The British army spokesman in Basra, Major Tom Holloway, told BBC News 24 that Saturday's artillery was fired from the British base at Basra airport, following a request from Iraqi ground forces, who had been fired on by the mortar team.
It was part of the support plan already in place, he said.
"I wouldn't describe it as an escalation. Of course, we haven't fired before, this is the first time, but this is something we were always ready to do."
On whether British troops could end up going back into the city, he said: "We are here supporting Iraqi forces, and I think they've said in the past they don't foresee us coming into the city.
"British forces here plan on a wide variety of contingencies, and it would be inappropriate of me to discuss future plans."
He said the Iraqi action in Basra was a "complicated operation".
"I think we need to be prepared for this to run for a while."
He also said artillery was used by the British troops "reasonably regularly", but this was the first time during the current fighting.
US jets also bombed two sites in the north of Basra during the day, targeting what it said were militant strongholds, according to the UK military.
The BBC's Adam Brookes, in Baghdad, said the US and British involvement in the fighting in Basra appeared to have started to grow in the last couple of days.
This meant the Iraqi armed forces were meeting much stiffer resistance than they expected, and were asking the coalition for help, he said.
"I would anticipate the British commanders will now be calculating - should these requests for help continue to come, just how much help they are able or willing to give an Iraqi operation that seems to be running into trouble."
Basra was taken by British forces in 2003. They withdrew from the city to the airport last autumn, and handed over security to Iraqi forces in December.