Locals in Basra have welcomed the extra security

BBC correspondents in Basra, Iraq's second city, reported the region was "swamped" with troops on Thursday.

Checkpoints were set up to perform searches on traffic in and out of the city, causing gridlock in some areas.

The sudden increase in security has raised questions of a fresh threat of attack, and coalition officials say they are "assessing" the situation.

Security 'appreciated'

British Warrior armoured vehicles with heavy guns have also been moved into position to guard the approaches to the coalition provisional authority headquarters in Basra.

Workers at the coalition provisional authority headquarters in Basra have confirmed they are "locked in" at the sprawling complex, with no movement allowed in or out.

The lock-down is expected to last until Saturday evening subject to a new security assessment.

Over 250 international staff from about eight countries work at the compound, based around one of Saddam Hussein's former palaces.

A spokesman admitted that the measures - in the first alert of its kind since early September - came in response to intelligence following the suicide bomb in Nasariyah.

Eighteen Italians died in Wednesday's suicide bomb attack in Nasiriya which killed at least nine others and injured many more.

I am a bit apprehensive, but I just want to do my job and make a difference Private Chris Ranaldi, 22

The headquarters in Basra supports reconstruction, liaison with ministries and political work in the south of Iraq.

Local residents are said to welcome the increase in security following Wednesday's attacks, despite some bemusement at the speed and scale of reinforcement.

Foreign Secretary Jack Straw has pledged more British troops to be sent to Iraq if they are needed.

He has warned attacks like the one on the Italian police headquarters indicated elements against the coalition are becoming "increasingly organised".

Deployment

The 1st Battalion The Royal Scots, based at Dreghorn Barracks in Edinburgh, is being sent to Az Zubaryh, south of Basra for six months on a peace support operation known as Operation Telic.

More than 350 soldiers will fly out from Brize Norton air base in Oxfordshire today to join 20 Armoured Brigade to support the British forces-led boost in security in the region.

Major Gene Maxwell said the mood among the troops was upbeat.

Boosted security has been welcomed by most locals

"I think everybody is quite excited," he said.

"It is an interesting, professional challenge and there is a healthy degree of concern and apprehension, of course.

"But we have got a good reputation for getting on with people and we had a good reputation in Bosnia, so I hope we can go out and build on that reputation in Iraq."

Private Chris Ranaldi, 22, from Livingston, said he was looking forward to helping the Iraqi people but was also nervous of what lies ahead.

He said: "I am a little bit apprehensive, but I just want to go out and do my job and make a difference."

Numbers

Mr Straw said British troop numbers had dropped to around 10,000 since the height of the conflict.

"They are kept under close review. Both Geoff Hoon (Defence Secretary) and the Chief of the Defence Staff (General Sir Michael Walker) - are constantly making judgements about whether force numbers are adequate," he said.

Japan has said it will postpone sending troops to join the US-led coalition in Iraq until next year because of the worsening security situation.

Although there are no official figures, estimates put the number of Iraqi civilians killed in attacks since May at about 200.

Mr Straw has stressed that of the 30 countries who have troops in Iraq, he has seen no reports that any of them are thinking of withdrawing their forces.