Islamic militants who fled Iraq during the war have returned to conduct major attacks, the senior US administrator there says.
Attacks on US soldiers have not relented
Paul Bremer said hundreds of fighters from the Ansar al-Islam group had slipped back into Iraq from refuge in Iran.
He was speaking days after a car bomb outside the Jordanian embassy in Baghdad killed at least 14 people.
In the south, UK troops have been struggling to contain rioters protesting at fuel shortages for a second day running.
Mr Bremer told the New York Times that intelligence reports suggested Ansar was planning "large-scale terrorist attacks" in Iraq.
"We have to be pretty alert to the fact that we may see more of this," he said.
The Bush administration has tied Ansar to al-Qaeda.
The Federal Bureau of Investigation (FBI) is expected to investigate the bombing attack on the Jordanian Embassy.
In the absence of claims of responsibility for the attack, speculation has focused on Ansar.
Mr Bremer told the New York Times it was also possible that Ansar al-Islam or another militant organisation had provided technical expertise on making car bombs to former Baathists who then carried out the attack.
In a related development, the Gulf television station al-Arabiya showed on Saturday a recorded statement by masked men promising to continue a guerrilla war in Iraq.
The men said they belonged to a previously unknown group, the Brothers and Sons of the Mujahideen.
The statements come amid continuing instability in Iraq.
Initial reports of Ahmed's arrest were premature
Four US troops were wounded in ambushes in the northern city of Kirkuk and Baghdad on Saturday, officers told AP news agency.
In the south, British troops have been struggling to control rioters in Iraq's second city, Basra, for the second day running.
Soldiers fired warning shots and armoured cars patrolled the streets as hundreds of demonstrators attacked vehicles and burnt tyres in protest at fuel shortages.
Meanwhile another senior member of Saddam Hussein's government is said to be in custody.
Former Iraqi interior minister Mahmoud Dhiyab al-Ahmad - number 29 on the American list of 55 most-wanted Iraqis - surrendered on Friday, according to US Central Command.
The Americans had already announced his capture a month ago, but now say that was an incorrect statement.
"We thought we had him, but we didn't and now we do," a military spokesman said.