Four American soldiers were wounded near the northern city of Mosul (search), the military said Tuesday, as U.S. officials urged aid groups to keep working in Iraq after homicide bombers struck the Red Cross (searchheadquarters and three police stations in Baghdad.

The homicide attacks Monday killed about three dozen people and caused foreign organizations to weigh their role in the insurgency-plagued nation. It was the bloodiest day in the Iraqi capital since the end of major combat six months ago.

Just hours after the attacks in Baghdad, two U.S. patrols were ambushed Monday night near Mosul, Iraq's third-largest city, which in the past has been relatively peaceful.

One soldier was wounded when insurgents attacked his convoy in southeastern Mosul and three others were injured, one seriously, when their patrol was attacked by rocket-propelled grenades and automatic weapons in the town of Tal Afar, just west of Mosul, the U.S. command said.

The brazen and deadly attacks in Baghdad, including a rocket barrage Sunday on a hotel where Deputy Defense Secretary Paul Wolfowitz (searchwas staying, attested to the spike in resistance by opponents of the American occupation.

The attacks also shook the confidence of international organizations which were taking part in the reconstruction of Iraq.

Secretary of State Colin Powell urged the Red Cross and other nongovernment organizations -- as well as foreign contractors and the United Nations -- to stay in Iraq.

"They are needed. Their work is needed. And if they are driven out, then the terrorists win," Powell said Monday in Washington.

Antonella Notari, chief spokeswoman at ICRC headquarters in Geneva, said no decision had been taken whether to evacuate non-Iraqi staff from Iraq. Twelve of the dead in Monday's attacks were killed in a car-bombing outside the Red Cross office in a quiet street in central Baghdad.

However, the German TV network ARD quoted the head of the ICRC delegation in Iraq as saying the evacuation of Red Cross personnel would begin Tuesday.

The three-story Red Cross headquarters in Baghdad was empty on the day after the bombing, with staffers being ordered to remain at home. All the windows were broken and large cracks were visible across the building's facade. Workers and residents were cleaning up the debris on the street and trying to drain a pool of water caused by a ruptured water main.

The attacks Monday occurred at the start of the Muslim fasting month of Ramadan and left scenes of broken bodies, twisted wreckage and Iraqis unnerved by an escalating underground war. The dead included eight Iraqi policemen, at least 26 Iraqi civilians and a U.S. soldier.

Iraqi policemen foiled a fourth attack on a precinct in the capital's eastern suburbs by snatching an attacker before he could detonate explosives installed in his car. The man set off a grenade, and shouted "Death to the Iraqi police! You're collaborators" when he was seized.

It is uncertain what groups carried out the attacks. In Baghdad, U.S. and Iraqi officials blamed foreign fighters intent on targeting those who cooperate with the American-led occupation. The captured would-be bomber was said to carry a Syrian passport.

In Washington, however, Pentagon officials said they believed loyalists of ousted President Saddam Hussein were responsible. President Bush said insurgents had become more "desperate" because of what he said was progress in Iraq.

Since Bush declared an end to major combat in Iraq six months ago, 113 U.S. soldiers have been killed by hostile fire, and about 1,675 have been wounded. U.S. forces come under attack an average of 26 times a day, and incidents have been on the rise since early September.

During a meeting Monday with U.S. Iraqi administrator L. Paul Bremer, Bush pledged to hunt down the "cold-blooded killers, terrorists" who are conducting the attacks. However, there has been little sign of progress in pursuing those responsible for a string of bombings during the last month.

"The more progress we make on the ground ... the more desperate these killers become," Bush said.

But Democrats immediately took issue with his assertion, leveling some of their sharpest criticism yet.

"Is the president arguing that the better things get in Iraq, the more dangerous it will become for American soldiers?" said Sen. John Kerry, a White House candidate and Vietnam veteran.