U.S. soldiers opened fire on a pickup truck that failed to stop at a checkpoint in central Ghazni (search) province, killing a man and two women and critically injuring two other people, the military said Sunday.

The deaths, the latest in a string of civilian casualties at the hands of U.S. forces in the past year, were likely to spark anger in a population that often views American soldiers as heavy-handed and overagressive.

The deaths occurred on a road in Ghazni province at about 9 p.m. Saturday when the pickup truck ran through a joint U.S.-Afghan military checkpoint, the military said in a statement Sunday. Soldiers searched the pickup but did not find any weapons.

It said the incident was under investigation, though it gave no indication any of the soldiers involved were believed to have acted improperly.

"Vehicle checkpoints play an important role in maintaining security in the area, providing security forces the opportunity to find wanted people and contraband, such as weapons, bombs and drugs," the statement said. "Vehicle occupants approaching checkpoints should stop and follow security personnel's instructions."

A critically injured man and woman and an uninjured infant were evacuated for medical treatment to Bagram Air Base (search), the U.S. military's main base in Afghanistan, the statement said.

Jawed Ludin, a spokesman for President Hamid Karzai (search), said the government was looking into the deaths and would have a statement later in the day. The U.S. military could not immediately be reached for comment to further explain the incident.

On Aug. 9, Lt. Gen. David Barno, the top U.S. commander, agreed to a 15-point plan to improve relations with Afghan civilians and officials after Karzai called him in for talks, a military spokesman said.

Local leaders have repeatedly complained of heavy-handed tactics by the U.S.-dominated coalition, especially during searches that sometimes involve air power and take place in the dead of night.

Rights groups warn that the muscular approach may have caused unnecessary deaths and stoked sympathy for Taliban rebels who continue to defy the currently 18,000-strong force under Barno's command.

The military says its critics forget that, more than two years after the hardline Islamic regime's ouster, Afghanistan remains a war zone. Taliban insurgents routinely target civilians, election workers and international aid groups. Hundreds of people have been killed in violence this year.

A U.S. air raid in Uruzgan province in January killed 11 villagers, including four children, Afghan officials charged. The U.S. military has maintained that five militants were killed in the raid

On Dec. 5, six children died when a wall fell on them during a nighttime assault on a complex in eastern Paktia province where the U.S. military seized hidden weapons caches.

The next day, nine children were found dead in a field after an attack by an A-10 ground attack on a village in neighboring Ghazni province.

Both attacks were aimed at wanted militants, but neither target was killed or detained.

American commanders vowed to review their procedures after the raids.

The U.S. military is also investigating several allegations that prisoners were abused in its jails in Afghanistan, including at least four deaths.