British military officials say paramilitary forces loyal to Saddam Hussein have opened fire on civilians attempting to leave Iraq's second city, Basra.
Conditions in the city are difficult
UK troops say they fired on the paramilitaries in response and tried to position themselves between the gunmen and the civilians.
It is unclear how many people tried to leave the predominantly Shia southern city, with some reports speaking of up to 2,000 while others say hundreds.
While there are no reports of deaths and an unconfirmed number of casualties, military officials said they were trying to get ambulances into the city, which has been encircled by UK troops for the past five days.
It has been suggested that the civilians may have been trying to get to food and water aid at points outside Basra - as they had done on previous days.
With water supplies cut by half and food in short supply, UN Secretary General Kofi Annan has warned that the city faces catastrophe if aid does not arrive soon.
The coalition is keen to be seen distributing aid, and hopes a massive shipment of emergency food, blankets, clothing and fresh water arriving in the southern port of Umm Qasr on Friday will signal that it is serious about humanitarian efforts.
UK and Iraqi tanks battled on Thursday
Iraqi Information Minister Mohammed Saeed Sahaf has meanwhile said in Baghdad that 116 people had been killed and 659 injured in Basra since the war began last week.
British forces have engaged military units leaving Basra at least three times this week, including a major tank battle on Thursday.
There have been several confused reports from the Basra region over the past few days.
On Tuesday UK sources said there was an uprising going on in the city, but coalition forces later conceded the situation on its streets were uncertain, while other reports suggested the city was quiet.
An Iraqi woman seeking aid in the south told the BBC's Kylie Morris that the people do not support Saddam Hussein, but neither do they welcome a takeover by US and UK forces.
Washington and London have openly been hoping for civilian uprisings against the Iraqi leadership.
Such an event in Basra - a city of 1.3 million people - would be a significant development.
But correspondents in the region say people are wary of rebelling after an uprising following the 1991 Gulf War was suppressed brutally by Saddam Hussein.
Many Iraqis felt they had been betrayed by the West after having been encouraged to revolt, they say.