United States Defence Secretary Donald Rumsfeld has accused Iran and Syria of allowing militants to cross their borders into Iraq.
Rumsfeld (left) met Bremer and other US officials in Baghdad
Mr Rumsfeld was speaking during a day-long visit to Baghdad to assess the security situation in the country.
Earlier a car bomb killed at least 13 people outside a police station in the northern city of Kirkuk.
Mr Rumsfeld has in the past accused Iran and Syria of harbouring militants - but both have denied the allegations.
"Syria and Iran have not been helpful to the people of Iraq", the defence secretary told journalists on Monday. "Indeed they have been unhelpful."
He added: "We know Iran has harboured al-Qaeda, we know they had people moving across the border. They were certainly aware of that."
Syria, however, denied that it was coming under pressure from Washington over the security situation in Iraq.
Syrian Prime Minister Naji Otari told the BBC that there had been some disagreements in the past but that these had been cleared up.
US officials in Iraq say militants influenced by al-Qaeda have overtaken supporters of former Iraqi leader Saddam Hussein as the main threat facing the American-led coalition.
"It's quite clear in the past three months we've seen a real step up on the part of the professional terrorists of al-Qaeda and Ansar al-Islam, conducting suicide attacks," US administrator in Iraq Paul Bremer said.
Ansar al-Islam is a local group said to be allied with Osama Bin Laden's network.
In February alone at least 250 people have been killed in Iraq - most of them policemen or police recruits.
Mr Bremer added that attacks like the one in Kirkuk were aimed at disrupting plans to give greater responsibility to Iraqi forces for maintaining security.
Thirteen people died and more than 50 were wounded in the attack.
A car sped through the entrance gates and blew up in the courtyard of a police station, US officials said.