US President George Bush has predicted that more foreign troops will join American forces in Iraq to help win "the continuing battle in the war on terrorism".
The body of UN envoy Sergio Vieira de Mello has been flown out of Iraq
Mr Bush said "al-Qaeda-type fighters" were moving into Iraq, but insisted the US would "stay the course".
He said he was working with the United Nations to seek broader international support - but he gave no sign he was prepared to relinquish more power to the UN.
Iraqi police said that earlier this week they arrested a group of Iranians who were planning a sabotage campaign in Baghdad.
A source within the Iraqi interior ministry told the BBC the Iranians had been handed over to the American military police for interrogation.
Speaking in Seattle, Mr Bush said a "foreign element" was moving into Iraq.
"They want to fight us there because they can't stand the thought of a free society in the Middle East," he said.
London and Washington want a UN resolution authorising a larger international force, following Tuesday's bomb attack on the UN headquarters in Baghdad.
But the UN Secretary General, Kofi Annan, has warned they face a "difficult" task to secure wider involvement in Iraq unless the UN is given more say.
Germany, France and Russia - three of the most vociferous opponents of the US-led invasion - are among those demanding a wider role for the UN.
None of the three are expected to send troops but several potential large contributors, such as India, Pakistan and Turkey, have refused to send soldiers without a stronger UN mandate.
"We do need and welcome more foreign troops into Iraq and there will be more foreign troops into Iraq," Mr Bush said.
"Those who hate freedom destroyed the infrastructures that we've been improving, so we'll get more people guarding then, and that'll help free up our hunter teams."
The BBC's Greg Barrow at the UN says many countries have expressed a willingness to assist in Iraq but they do not want the stigma of serving under an occupying force which has yet to win over the hearts and minds of the Iraqi people.
The body of the UN's envoy to Iraq, Sergio Vieira de Mello, who was among more than 20 killed in Tuesday's apparent suicide attack, is being flown to Brazil for a wake, before his funeral in France.
Vieira de Mello's interim replacement in Iraq has been named as Ramiro Lopes da Silva, a fellow Brazilian.
Mr Annan said the UN was largely counting on the US-led coalition to provide its staff with security in Iraq.
PLANNED MULTI-NATIONAL FORCE FOR IRAQ
Multi-national force: This will be the largest mission of its kind to take place without UN co-ordination or central Nato command
Cost: US reportedly expected to pay partner nations a total of more than $200m in airlift and support costs
Twenty-seven nations have sent troops:
Albania, Azerbaijan, Bulgaria, the Czech Republic, Denmark, the
Dominican Republic, El Salvador, Estonia, Georgia, Honduras,
Hungary, Italy, Kazakhstan, Latvia, Lithuania, Macedonia, Mongolia,
the Netherlands, Nicaragua, Norway, Poland, Romania, Slovakia, South
Korea, Spain, UK, Ukraine
Several nations' troops are expected:
Moldova, Philippines, Portugal - Japan and Thailand have however expressed reservations