The US administrator in Iraq has said the bombing of the UN office in Baghdad marks a new front in the war against terror.
Bremer said there could never be 100% security
Paul Bremer was speaking in the city as Brazil paid its last respects to UN envoy Sergio Vieira de Mello, killed along with at least 22 others.
He said that while there was little evidence of who had carried out the attack, about 100 foreign militants had recently infiltrated Iraq.
Unknown militants ambushed UK troops in Basra on Saturday, killing three, while ethnic clashes in Kirkuk have left nine people dead.
Mr Bremer, who wept over the coffin of Mr Vieira de Mello before it left Baghdad on Friday, said the UN bomb was of a size not seen before in post-war Iraq.
"It is now unfortunately the case that Iraq has become one of
the fields of battle in this global war," he told reporters.
He said there were two kinds of "foreign terrorists" now entering Iraq: those who travelled from places like Syria, the Sudan or Yemen, and those who were part of the militant group Ansar al-Islam, which has been linked with al-Qaeda.
"You have to be willing to go on the offensive against terrorism
- kill them before they kill you," the US administrator said.
US President George W Bush pledged on Saturday to "continue this war on terror until all the killers are brought to justice".
In Kirkuk in the north, ethnic Kurds and Turkmen tribesmen fought lethal battles with guns and grenades after a dispute which erupted on Friday night, apparently over a newly reopened Turkmen shrine.
US troops are struggling to restore order in Kirkuk
The ambush in the southern city of Basra brought UK combat losses since the official end of major combat on 1 May to 10. US losses for the same period exceed 60.
In the Iraqi capital, it has emerged that the British Embassy has been evacuated because of the threat of an attack.
UN staff have been resuming work in the city out of tents and shipping containers, set up next to the ruins of their headquarters in the Canal Hotel.
Jan Egeland, the new UN under secretary general for humanitarian affairs, said there would be fewer staff, they would travel less and they would be hampered by strict security procedures.
Mr Bremer said security was under review and he believed that the bombing had galvanised the will of the world to help.
The US administrator also said he shared the frustration of Iraq's Governing Council at the failure to restore essential services to pre-war levels.
In the absence of hard information about those behind Tuesday's apparent suicide attack, a man purporting to represent an Iraqi Islamic militant group called Mohammed's Army has claimed responsibility.
An unidentified masked man appeared on Lebanese satellite TV on Saturday to say the group had attacked the offices because US
intelligence officers were present there.
A Dubai-based TV channel, al-Arabiya, earlier carried a similar claim from the previously unknown group.