A US commander in Iraq has said the war is not over, with coalition forces still fighting organised remnants of the deposed regime.
General McKiernan said there may be more battles
Lieutenant-General David McKiernan said more forces may have to be deployed on combat operations around the country.
US units have been come under nearly daily attacks which General McKiernan blamed on "thugs under [Saddam Hussein's] regime" who have "no future in this country".
His comments came as UK Prime Minister Tony Blair made the first visit to Iraq by a Western leader since the toppling of Saddam Hussein.
Mr Blair told British troops in Basra that they took part in "a momentous and mighty act" in liberating the country.
US military officials on Thursday said an American soldier had been killed by "hostile fire" - the fifth such death in a week.
"The war has not ended," said General McKiernan, the commander of US ground troops in Iraq.
"Decisive combat operations against military formations have ended, but these contacts we're having right now are in a combat zone, and it is war, and they are members of [Saddam Hussein's] regime that must be removed."
He said more soldiers might be deployed on combat operations though he did not believe the recent escalation of attacks would become a trend.
His assessment was echoed by the commander of US marines in Iraq, Major-General Jim Mattis, who said there were still some "regime thugs" operating, but "they are not significant and are relatively easy to take out".
The BBC's Pentagon correspondent, Nick Childs, says the continuing level of attacks is causing concern among politicians.
He says military chiefs may not be able to bring units home as soon as they had wished, and the battle-hardened Third Infantry Division, which had been hoping to head home, looks as if it will have to stay on.
There are now more troops in Iraq than at the height of battle but even so the force is still relatively small for such a large country when compared with peacekeeping missions in the Balkans.
Motives for war
Mr Blair said history would look back on the Iraq conflict as "one of the defining moments of the century".
He described the coalition victory as an opportunity for broader change from the Gulf to Israel and the Palestinian territories, to Iran and Syria.
The visit was the first by a Western leader since the end of the war
But the BBC's Richard Miron in Basra says questions linger locally and internationally over the motives for the war and for what will happen now.
Many Iraqis in the south are frustrated at what they see as British inaction over the poor security situation, our correspondent adds.
Mr Blair told troops: "I know there were a lot of disagreements in the country about the wisdom of my decision to order the action.
"But I can assure you of one thing - there is absolutely no dispute in Britain at all about your professionalism and your courage and your dedication and not just in the way you won the war, which was extraordinary, but the way you are conducting the peace, which is remarkable."
The US soldier who was shot and killed on Thursday was attacked while travelling on a main supply route, military officials said.
He became the 20th US serviceman to die in fighting or accidents in Iraq since 1 May, the date President George W Bush declared the war effectively over.
A police station was destroyed in the western town of Hit
There have also been reports of protests turning violent in the town of Hit, about 200 kilometres (120 miles) west of Baghdad.
A police station requisitioned by US forces came under attack and was set on fire, forcing soldiers and Iraqis to retreat.
One report suggested the attack was a response to intrusive house-to-house weapons searches by Iraqi police backed by US troops.
Angry residents surged into the streets, burning police cars and throwing stones and hand-made grenades at US forces, the Reuters news agency said.