A rocket-propelled grenade slammed into a US military ambulance Thursday, killing one American soldier and wounding two others, the latest in a series of attacks on US personnel or their offices.

The ambulance was transporting a wounded American soldier to a medical facility when it came under fire on a highway about 20 miles south of Baghdad.

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

The wounded were taken to the 28th Combat 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.

Three mortar shells exploded Tuesday outside a coalition-run humanitarian aid office in the town of Samarra, north of Baghdad, killing an Iraqi bystander and wounding 12 others, hospital officials and US officers in the town said Thursday. No American forces were hurt. The military initially said the attack happened Wednesday.

Attackers also fired a rocket-propelled grenade that struck a US tank in Samarra, said Sgt. Steven Stoddard with the Army's 4th Infantry Division. Another tank fired back, killing one attacker, while the second was captured, Stoddard said. There were no American casualties.

In west Baghdad, an Army truck was hit by what witnesses said was a rocket-propelled grenade. The torn-apart truck sat burning on the edge of the highway.

Witnesses said there were casualties, but US military police at the scene said the vehicle broke down earlier and was set on fire after being left alone while soldiers prepared to remove it.

The mortar rounds in Samarra, 75 miles north of Baghdad, exploded outside the Civil Military Operations Center. US soldiers heard three explosions and asked local police to investigate, said a US Central Command statement.

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

The office coordinates between the military and civilian agencies in the area.

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 US 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, killed by US troops, 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 men were shot after the protest turned violent, the US military said.

"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. US 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.

As the US military grappled with an increase in guerrilla attacks, 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 US 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 US-led occupation.

US forces said Wednesday they captured Saddam Hussein's top aide and presidential secretary, a man who American officials believe knows the fate of the deposed Iraqi leader and has information about banned weapons.

Abid Hamid Mahmud al-Tikriti was No. 4 on the US most-wanted list of Iraqi leaders, behind only Saddam and sons Qusai and Odai.

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