By Hilary Andersson BBC correspondent in southern Iraq

Many of Basra's inhabitants have been trying to move in and out of the city

British troops are moving in closer to the city centre of Basra.

They are on three sides of the city and we have seen the 16 Air Assault Brigade move closer down toward the city from the north.

A lot of fighting is taking place alongside four bridges which cross a waterway just to the west of Basra.

The troops on the ground have been told by senior commanders that they must not hit civilians if they can possibly avoid it.

But British troops have been drawn into urban fighting because they are firing their biggest guns - AS 90 artillery - straight into the centre of Basra and also to targets on the outskirts.

We have been told that amongst the kinds of ammunition being fired from these huge guns are effectively cluster bombs, called L20 bomblet rounds.

There are being fired, we are told by the military, only into open areas on the outskirts of Basra - not into the city centre.

But these are rounds that carry lots of little bombs which scatter over a big area.

The British say they are using them to destroy large numbers of Iraqi troops only when they move into open ground.

It is the first time these L20 bomblet rounds have been used at all.

The British military have been very careful to point out that they are not being used in the city centre.

Civilian shelter

I am sure parents will be telling children not to go into areas where rounds like this have been fired - that they would be trying to keep their families away from any of the centres that have been heavily bombarded.

People in Basra have barely slept in days. They have had these enormous rounds being fired at them over their heads in the middle of the night

But we are talking about a city of one and a half million people, and a lot of these people have been trying to move in and out of the city. A great deal of the fighting has been happening along the fringes.

British troops have been quite clear about it. They have said: "This is a real problem for us - the Iraqis are taking shelter with their military equipment, their T55 tanks, and so on, inside civilian structures."

Another tactic the British use is to light up the areas where they believe the Iraqis are, with illumination rounds.

They wait for the Iraqis to move out of the places where they are taking shelter and then they try to destroy them.

But clearly, it is really difficult fighting terrain because the British have been drawn into urban warfare.

Aid effort

The British have not yet got themselves into a situation where they can get aid supplies - water or anything - into central Basra because they do not control the city.

The Iraqis are still very much in charge. So there is a situation where people do need water quite badly.

The predominant message from the people of Basra over the last two days when we have talked to people coming in and out is that their main concern is not so much the humanitarian situation, but the fighting.

They have barely slept in days. They have had these enormous rounds being fired at them over their heads in the middle of the night.

They find half of their city has been lit up with illuminating rounds.

The Iraqis are taking shelter in-between civilian houses and using those houses as places to fire from.

This means civilians could be in the line of fire that comes back from the coalition forces.

So no matter how well intended the British troops might be, the civilians are trapped in the fighting and they are under severe pressure.