At least one American soldier was killed and two others were wounded Thursday in a rocket-propelled (searchgrenade attack on a U.S. military ambulance in southwest Baghdad, the U.S. military said.

The ambulance was transporting a wounded U.S. soldier to a medical facility when it was hit at about noon while on a highway in al-Iskandariyah, about 20 miles south of Baghdad.

The wounded soldier who was being transported in the ambulance was not the one who was killed, said Capt. John Morgan, a U.S. military spokesman in Baghdad. The casualties were members of the Army's 804th Medical Brigade and their identities were being withheld pending notification of relatives.

The wounded were taken to an Army support hospital in southwest Baghdad. It was not immediately clear if the ambulance was traveling as part of a convoy or if fire was returned.

The attack, which occurred around noon, was the third reported violent incident against U.S. personnel or their offices in the last 24 hours.

Iraqi attackers also fired a rocket-propelled grenade that struck a U.S. tank in Samarra, said Sgt. Steven Stoddard with the Army's 4th Infantry Division (search). A second tank fired back, killing one attacker and wounding another, Stoddard said. There were no American casualties.

Earlier in the day, an Army truck was hit by what witnesses said was a rocket-propelled grenade. A U.S. military column apparently came under attack, one vehicle was disabled and the torn-apart truck sat burning on the edge of the highway.

Witnesses said there were casualties and that three soldiers were quickly pulled out of the vehicle by the U.S. military. However, U.S. military police at the scene said the vehicle had broken down earlier and was set on fire after it was left alone while soldiers prepared to remove it.

On Tuesday, three mortar shell exploded outside the Civil Military Operations Center (search), a coalition-run humanitarian aid office, in Samarra, 75 miles north of Baghdad. U.S. soldiers heard three explosions and asked local police to investigate, said a statement from U.S. Central Command (search).

The statement said Samarra police found the injured and killed and that soldiers were unable to find the attackers.

The U.S. military said one Iraqi bystander was killed and 12 others were wounded. No American forces were hurt.

It was the latest of several attacks targeting Iraqis working with U.S. forces. The office coordinates between the military and civilian agencies in the area.

An American soldier was also killed Wednesday by two gunmen who walked to the propane distribution point that his squad was guarding and shot him at close range. A second soldier was wounded pursuing the gunmen, who escaped in a waiting car.

On Monday, a sniper killed a U.S. soldier patrolling the dark streets in his vehicle with a single shot in the back.

The U.S. Army has said it believes guerrillas loyal to Saddam Hussein are behind a recent wave of attacks on U.S. forces in Baghdad and restive areas to the north and west, reported Reuters.

Meanwhile, scores of angry mourners fired Kalashnikov assault rifles into the air and shouted curses at the United States during a procession Thursday for two Iraqis who were shot dead by U.S. troops at a protest by disgruntled former army officers.

Shouting "Death to Bush!" and "Revenge," mourners marched with the body of 32-year-old former Iraqi army officer Tareq Hussein Mohammed from his house in northern Baghdad to a mosque.

Mohammed was one of two men shot outside the gate of the coalition headquarters in Baghdad during a demonstration of ex-soldiers demanding their salaries. The U.S. military said the men were shot after the protest turned violent.

"Abu Soheib, come back to us," wailed his wife Soheir, using his nickname. "Now there is no salary, and no man."

As neighbors saw the coffin arriving at his house from the morgue, they fired their weapons into the air for more than 15 minutes at a time in a deafening, frenzied display of defiance. U.S. troops have prohibited people from shooting their weapons in the streets.

In Iraq, shooting into the air is also a sign of respect for the dead.

"Iraqis are going to kill Americans. We are going to take revenge for Tareq's blood," said Salwa Mohammed, a relative of the slain man.

Black-clad women at the house sat on the floor and wailed.

The United Nations reported that an increase in power outages in the capital of Baghdad was caused by sabotage to Iraqi power lines. The United Nations also reported that humanitarian assistance vehicles were being fired upon, along with those of the American military.

Meanwhile, the U.S. Army's Baghdad radio station began broadcasting appeals for Iraqis -- including ex-military personnel -- to join the civilian police force in Baghdad and Fallujah. Some say the upsurge in violence is at least partly due to the huge number of former soldiers and officers of the ousted regime who lost their jobs.

Iraqi cities have been on edge since Sunday, when coalition forces began house-to-house searches in Baghdad for banned weapons and suspected activists trying to undermine the U.S.-led occupation.

Iraqi security officials working with the Americans say regional leaders are directing the attacks by Saddam loyalists, former soldiers, Sunni Muslim radicals and non-Iraqi "holy warriors."

In Washington, Defense Secretary Donald Rumsfeld told reporters Wednesday at the Pentagon that groups of 10 to 20 people were behind the attacks on soldiers, not "large military formations."

Attacks and accidents have killed about 50 American troops -- including about a dozen from hostile fire -- since major combat was officially declared over on May 1. Between March 20, when the war started, and May 1, 138 Americans died from accidents or hostile fire.

The commander of the 4th Infantry Division, Maj. Gen. Ray Odierno, called the losses regrettable but "militarily insignificant."

Fox News' Gregg Jarrett and The Associated Press contributed to this report.