The American civil administrator in Iraq says Iraq has become one of the battlefields in the war on terrorism, following Tuesday's bomb attack on the United Nations headquarters in Baghdad that killed 23 people. The remarks by Paul Bremer came as three British soldiers were killed in an attack in the southern Iraqi city of Basra.

The head of the Coalition Provisional Authority said the attack on the U.N. headquarters in Baghdad was the work of terrorists and added that preventing such attacks poses the greatest difficult challenge to the intelligence community.

"Iraq has become a new field of battle in this worldwide terrorist fight," said Paul Bremer. "And therefore we will continue to refine our intelligence. But there is no such thing as 100 percent security against terrorism. And what we have to do is raise the threshold as high as we possibly can for terrorism, make it as difficult as possible for them."

Speaking at his first news conference since the attack, Mr. Bremer, a counter-terrorism expert, said it was too early to say who was responsible. But he said there were three possible hypotheses.

"One is that it was done by members of the former regime, for a variety of reasons," he said. "Another is that it was done by foreign terrorists, of which there are several varieties around. The third, quite obviously, is that it was done in some form of cooperation between the two. As far as I can tell from the facts as I have seen them so far, all three of these hypotheses are still at least worth pursuing."

The remarks came as three British soldiers were killed by unidentified gunmen in the southern city of Basra. Officials said their vehicle crashed after coming under fire while on a routine patrol. A fourth soldier and two Iraqi civilians were reportedly injured in the incident.

Mr. Bremer said that despite what he called a grim week, reconstruction efforts continued across Iraq. Many U.N. staff members returned to work Saturday, setting up offices in tents on the damaged U.N. compound and in temporary buildings in various parts of the city.

Workers also continued to clear rubble from the site of last Tuesday's attack. Officials say two U.N. workers are still unaccounted for and the bodies of an unknown number of visitors to the facility are still under the wreckage.

The remains of the United Nation's senior official in Iraq who died in the attack, Sergio Vieira de Mello, were flown to Brazil after an emotional ceremony at the international airport here.