Gunmen opened fire on a van filled with police recruits south of Baghdad (search) on Tuesday, killing nine, and assailants shot and killed two policemen -- twin brothers -- north of the capital.

The slayings were the latest to target police and other Iraqis who work with the U.S.-led occupation.

Early Wednesday, attackers fired a rocket that struck the Sheraton Hotel, where foreign contractors and journalists stay. Security guards said there were no reports of casualties.

The rocket hit a sixth-floor ledge and the explosion left the lobby strewn with glass, said security guard Rabah Nasser.

U.S. soldiers protect the Sheraton, which is ringed by a concrete blast wall to ward off bombing attempts. A U.S. military spokesman said he had no information on the blast.

Tuesday's roadside attack in the south took place between Musayeb and Hillah (search) when a car pulled in front of the minibus and assailants sprayed it with small arms fire, police in Hillah said.

A U.S. military official confirmed that nine people died and said two were wounded. Iraqi police said one wounded trainee survived.

In the northern city of Kirkuk (search), gunmen in a car killed two policemen and wounded two others, police Capt. Abdul-Salam Zangana said. He identified the slain victims as twin brothers Ahmed and Mohammed Kadhim, killed as they were parking their car in a main square and as worshippers left a nearby mosque.

In Ramadi, west of Baghdad, Iraqi police fired shots to disperse a violent protest against Israel's assassination of Hamas founder Ahmed Yassin in Gaza City on Monday.

Iraqi police fired in the air after protesters burned two police cars and two hand grenades were thrown at the governor's office, witnesses said. Television footage showed U.S. soldiers remaining behind at the building, protected by concrete blast barriers, as police with assault rifles moved down the street to disperse the crowd. At least two police and three protesters were wounded.

Muslim clerics in Ramadi, where support for the anti-U.S. insurgency is strong, had urged followers to protest the slaying of Yassin, the spiritual leader of the Palestinian militant group.

In the northern city of Mosul, insurgents fired mortar rounds at a barracks housing soldiers of the U.S.-trained Iraqi Civil Defense Corps, killing two civilians and injuring six, the U.S. military said.

Also in Mosul, a U.S. soldier from the 3rd Brigade of the 2nd Infantry Division died in a "non-combat-related shooting" on Monday, the military said. An investigation was under way. The soldier's name was withheld pending notification of next of kin.

At Abu Ghraib prison on the outskirts of Baghdad, the U.S. military released 272 detainees who had been picked up in security sweeps. The men, many of them bearded and wearing Arab robes or tracksuits, appeared to be in good health.

Two days earlier, 168 prisoners were released, said Lt. Col. Craig Essick of the 16th Military Police Brigade out of Fort Bragg, N.C. He estimated there were 5,500 to 6,000 security detainees at the prison and the average prisoner spends three to six months in jail. A military panel meets daily to review cases.

In Baghdad, Les Brownlee, the acting Secretary of the Army, said commanders are trying to relieve the strain on U.S. troops in Iraq in case the tough deployment deters Americans from volunteering for the armed forces or extending tours of duty.

"In the long run there could be some impact," Brownlee told reporters at Saddam Hussein's former Republican Palace, headquarters of the U.S.-led occupation.

Brownlee said the Army is already reacting to the rigors of Iraq by planning to add 30,000 new troops -- 10 new combat brigades -- to its ranks over the next several years. In Iraq, the aim is to use as many new Iraqi soldiers as possible for jobs currently handled by U.S. troops.

For American forces, Iraq is a dangerous assignment where comforts are few. Restrictions on the force, including no drinking or recreational trips off base, make the assignment a dreary one. Since the invasion a year ago, 584 U.S. troops have died -- 396 from hostile action.

The next troop rotation begins in the fall. The Army is in the process of bringing 110,000 troops into Iraq, replacing the current U.S. force of 130,000.

Meanwhile, a spokesman for a member of the U.S.-appointed Iraqi Governing Council said the council will investigate alleged corruption in the U.N. oil-for-food program.

Entifadh Qanbar, spokesman for council member Ahmad Chalabi, said the council will hire international legal and auditing firms as well as specialized Iraqi firms to conduct the probe.

On Monday, the United Nations said Secretary-General Kofi Annan would give the Security Council details about a planned independent commission to investigate claims of corruption in the program.

Diplomats, officials and companies from around the world allegedly collected millions of dollars in illegal profits from the program, which allowed Iraq to sell some of its oil to pay for food during the years of economic sanctions. The program ended in November.