His visit was hastily arranged after Prime Minister John Howard declined an invitation to attend a war summit with President George W. Bush and Britain's Prime Minister Tony Blair last week.

Downer is expected to hold talks with US Secretary of State Colin Powell and National Security Adviser Condoleezza Rice and with UN officials.

High on the agenda will be the reconstruction of post-war Iraq as well as humanitarian aid for the Iraqi people, Downer said of his Washington talks.

He acknowledged Iraqi tactics such as suicide bombings could extend the military campaign.

Four US soldiers were killed after a car exploded at a checkpoint near the city of Najaf, south of Baghdad, killing the driver and four US soldiers who were searching the vehicle.

But Downer denied the Iraqi war strategy would change coalition military tactics or lessen the emphasis on minimising civilian casualties.

"I know that puts us at some disadvantage," he said.

"It does mean that the war will take a little longer than would otherwise have been the case."

Australia has committed 2,000 military personnel to the war.

Defence Minister Robert Hill said the guerilla tactics were another challenge for coalition forces and may have caught the United States off guard.

"I think they underestimated the role of the militias within the smaller cities and towns," Hill said.

"They've fought hard and they certainly have resulted in some delays of progress."

WAR.WIRE