US forces in Iraq are holding 19 suspected members of the al-Qaeda network, the US administrator in Iraq has said.
Attacks on US troops have become more sophisticated, the US says.
The suspects are among 248 foreign fighters being held in Iraq, Paul Bremer told reporters in Washington.
The largest number of fighters - 123 - came from Syria, with large numbers coming from Iran and Yemen, he said.
The United States has blamed some of the attacks against Western targets in Iraq on foreign fighters who have apparently managed to infiltrate the border.
Mr Bremer said most of the "terrorists" were getting in via "ratlines" from Syria.
The US has repeatedly accused Syria of blocking US efforts to restore order in Iraq - charges that Syria denies.
The flow of fighters into Iraq was the biggest obstacle to the peaceful reconstruction of Iraq, Mr Bremer said. But he insisted the fighting had not stopped the reconstruction efforts, which were seen as crucial to the US-led war on terror.
"We don't want Iraq to become a breeding ground for terrorism in the future," Mr Bremer said.
'Return of Ansar'
The US administrator said he did not have the nationalities of all the foreign fighters captured in Iraq.
When pressed on how he knew 19 of the detainees were al-Qaeda members he told reporters: "That's been a matter that has come out in their interrogations or in their documents."
Mr Bremer said some of the suspects were members of Ansar al-Islam, an al-Qaeda-linked group whose base in northern Iraq was destroyed early in the Iraq conflict.
Possibly several hundred Ansar members had managed to regroup and re-enter Iraq, he said.
Mr Bremer has been testifying in the US Congress this week to argue in favour of President Bush's request for $87bn for spending on Iraq.
The money would mostly be for military costs in Iraq and reconstruction of the country's collapsed infrastructure.