That followed decisions by the International Committee of the Red Cross and Medecins Sans Frontieres to reduce their staffs.
Insurgents kept up their attacks against U.S. and other targets, blasting a freight train carrying military supplies near Fallujah west of Baghdad.
An improvised bomb caused no casualties but set four containers ablaze and sparked a frenzy of looting by residents who carried off computers, tents, bottled water and other supplies.
A soldier from the 2nd Brigade of the 101st Airborne Division was slightly injured early Thursday when a bomb exploded near a U.S. convoy in the northern city of Mosul, the military said.
An explosion shook Baghdad's old quarter late Thursday, triggering a large fire. Gunfire broke out in the area as U.S. helicopters flew overhead. There was no immediate information about damage or the cause of the blast on al-Motanabi street, known for rows of bookshops and antique stores.
The exodus of humanitarian workers came despite assurances by top U.S. administration officials — including President Bush — that the security situation in Iraq was steadily improving.
It also followed a personal appeal by Secretary of State Colin Powell to the international Red Cross to remain in Baghdad because "if they are driven out, then the terrorists win."
In other developments:
U.S. forces are now suffering an average of 33 attacks a day — up from about 12 daily attacks in July. A total of 117 American soldiers have been killed in combat since May 1 — when Mr. Bush declared an end to major fighting — or slightly more than the 114 soldiers who died in invasion that began March 20.
The violence continued with an apparent assassination attempt Wednesday night against an aide to Iraq's most influential Shiite Muslim cleric, Grand Ayatollah Ali Hussein al-Sistani, spiritual leader of most of Iraq's Shiite majority.
Sistani's office in the city of Najaf said Thursday they had no details on the attack on the cleric in the nearby city of Karbala and would not give his name. The Baghdad newspaper al Zaman identified him as Abdel Mehdi al Karbali.
In Tikrit, U.S. soldiers raided six houses after receiving "reliable intelligence" that the inhabitants were helping to establish a "new terrorist network in Tikrit and planning terrorist attacks against coalition forces," Lt. Col. Steve Russell told The Associated Press.
Four key suspects were detained, and 10 others were taken in for questioning, Russell said. Coalition troops discovered false identification cards and multiple fake license plates with "official government stickers" in one of the houses, he added.
The U.N. decision to pull its remaining international staff out of Baghdad was announced on Wednesday, two days after a deadly suicide car bombing at the Baghdad headquarters of the Red Cross.
"We have asked our staff in Baghdad to come out temporarily for consultations with a team from headquarters on the future of our operations, in particular security arrangements that we would need to take to operate in Iraq," U.N. spokeswoman Marie Okabe said.
She said it was not an "evacuation" and staff in the north would remain.
Okabe declined to give more details but about 60 U.N. staff members were believed to be in Iraq, including some 20 in Baghdad, after Secretary-General Kofi Annan ordered most others out in late September.
The United Nations scaled down its staff following the Aug. 19 truck bombing at its Baghdad headquarters that killed 23 people, including the top U.N. envoy to Iraq, Sergio Vieira de Mello, and a smaller blast near the U.N. offices last month.
The Red Cross said it would remain in Iraq but would scale back the number of international staff — now numbering about 30 — and increase security for those who stay. The agency has 600 Iraqi employees.
Asked on Thursday about U.S. pleas for the ICRC to stay put, the organization's Baghdad spokeswoman Nada Doumani said: "The ICRC will take its decision independently as it always had."