A number of British servicemen have died in a helicopter crash in southern Iraq, the Ministry of Defence has said.
Police in Basra told reporters the aircraft crashed into a city centre house after being hit by a rocket.
But new Defence Secretary Des Browne said the cause of the crash remained unclear, and he offered his sympathies to the families of those who had died.
Violent clashes in which some people were injured followed the crash, and an 8pm curfew was imposed by Iraqi police.
Basra police later said five people, including two children, were killed and nineteen others were wounded after Iraqis clashed with British forces.
Gen John Cooper, commander of British forces in Iraq, said troops did not fire directly into the crowds but fired live rounds at targets threatening them.
But an Army spokesman could not confirm claims that people had died in the disturbances.
Basra police said four people were killed in subsequent clashes
Mr Browne, who has only been in the job since Friday, said he was "deeply saddened" by news of the crash, but warned against speculation over what had caused the incident.
"The situation on the ground is still developing and facts are still coming in. We must be careful to allow those investigating the incident to do their job."
Mr Browne said troops were helping the Iraqi emergency services to secure the crash-site to help ensure a "thorough investigation of all possible causes of the incident".
The defence secretary paid tribute to the "courage and fortitude" of British forces in helping to build a "emerging democracy" for the Iraqi people.
In other developments:
- A suicide bomber wearing an Iraqi army uniform entered an Iraqi army base in Tikrit and detonated an explosives belt, killing three army officers
- Two Iraqi policemen were injured by a roadside bomb in the northern city of Mosul
- In Baghdad, two children were killed and a woman injured when a mortar landed on their house in the north of the city
Video footage from Iraqi television showed orange flames and large plumes of black smoke curling into the sky shortly after the crash.
Iraqi youths threw stones at British troops after the crash
British troops were seen running through the streets firing shots into the air as hundreds of Iraqis gathered, many waving their arms and throwing stones.
The footage also showed a Warrior tank being hit by home-made petrol bombs hurled by people in the crowd.
One of the petrol bombs caught alight on the top of the tank and fire extinguishers were seen trying to put out the flames.
Major Sebastian Muntz, in Basra, said the scenes in the city were "horrible", but claimed the situation for British troops there had got much better recently.
Defence analysts said British soldiers in the area were facing an increasing threat.
Ex-Commander of UK forces in Bosnia, Colonel Bob Stewart, said there was an "escalating situation" in Basra.
The BBC's Jim Muir in Iraq said if confirmed, it would be the first time a British military helicopter had been shot down in the area.
More than 100 British service personnel have been killed in Iraq in total.
The Liberal Democrats said it was an "appalling incident" and called for a "clear exit strategy" for British forces from the area.
The MoD said it was not the right time to be discussing strategy, saying their efforts were concentrated on dealing with the situation on the ground.
A hotline has been set up for families seeking information: 08457 800 900.