KABUL: A suicide bomber on a motorcycle killed 12 civilians in southern Afghanistan, as police fired Thursday on a crowd of rock-throwing protesters in the west who were angry about civilian deaths they blame on American bombing runs.

U.S. and Afghan investigators were examining the site of the alleged bombing deaths, but it was unclear how soon they would have a report.

In southern Helmand province, a suicide attacker on a motorbike set off his explosives Thursday near an international military vehicle in a market area of Gereshk district. The explosion killed 12 people and wounded at least 32 more, said Daud Ahmadi, a spokesman for the governor.

Ahmadi said he did not have information on whether there were any casualties among the international troops in the vehicle. A spokesman for NATO forces confirmed the incident but said he did not yet have any information on whether there were troop casualties.

In the capital of western Farah province, a protest erupted into a melee outside the governor's office. One protester was wounded by a bullet and five more suffered other injuries after they tried to storm the main government building there, said Gul Ahmad Ayubi, a health department official in the province.

The crowd of about 150 people were protesting the deaths of civilians in the villages of Ganjabad and Gerani, said Belqis Roshan, a provincial council member. The international Red Cross and local officials said the people were killed by American bombs, though the U.S. military said that may not have been the case.

Mohammad Nieem Qadderdan, a former district chief of Bala Buluk who visited the site of Monday's battle, said 100 to 120 people were killed. If 100 civilians died in the fight, it would be deadliest case of civilian casualties since the 2001 U.S.-led invasion. The Red Cross said women and children were among dozens of dead.

On Wednesday, Secretary of State Hillary Rodham Clinton said that the U.S. "deeply, deeply" regretted the loss of innocent life while opening a meeting with the presidents of Afghanistan and Pakistan at the State Department.

Afghan President Hamid Karzai has long pleaded with the U.S. to minimize civilian deaths during its operations, contending that such killings undermine support for the fight against the Taliban. Karzai quickly ordered an investigation into this week's violence.

The rising death toll also threatens Karzai's standing with the public as elections approach in August.

At least 18 presidential candidates had registered by midday Thursday, including former Finance Minister Ashraf Ghani and former Foreign Minister and Northern Alliance member Abdullah Abdullah. They are considered two of Karzai's stronger challengers.

"If you think that there is a need for a new start, we are at your service," Ghani told reporters after submitting his documents Thursday. Candidate registration closes on Friday.

Meanwhile in Farah, the team of investigators headed by a U.S. brigadier general was on the ground Thursday, said Capt. Elizabeth Mathias, a U.S. military spokeswoman. Afghan military and police officials were also part of the team. Mathias said she did not yet have information on what the team found.

But Gen. David McKiernan, the top U.S. and NATO commander in Afghanistan, voiced doubts about whether the deaths were due to an American airstrike.

McKiernan said U.S. military personnel had come to help Afghan forces who may have been ambushed by Taliban militants Sunday. He said the Taliban beheaded three civilians, perhaps to lure police.

"We have some other information that leads us to distinctly different conclusions about the cause of the civilian casualties," McKiernan said. He would not elaborate but said the United States was working with the Afghan government to learn the truth.

A senior U.S. defense official said late Wednesday that Marine special operations forces believe the Afghan civilians were killed by grenades hurled by Taliban militants, who then loaded some of the bodies into a vehicle and drove them around the village, claiming the dead were victims of an American airstrike.

A second U.S. official said a senior Taliban commander is believed to have ordered the grenade attack. The officials spoke on condition of anonymity because they weren't authorized to release the information.

Two other senior defense officials said the grenade report comes from villagers interviewed by U.S. investigators who went to the site, but there is no proof yet that the report is right.

Reto Stocker, the head of the international Red Cross in Afghanistan, blamed an airstrike for the death of his organization's volunteer and 13 members of his family who were sheltering inside a home.

Tribal elders called the Red Cross during the fighting to report civilian casualties and ask for help, Stocker said.

Villagers said they gathered children, women and elderly men in several compounds near the village of Gerani to keep them away from the fighting, but the compounds were later hit by airstrikes.

Provincial authorities have told villagers not to bury the bodies, but instead to line them up for the officials conducting the investigation, Qadderdan said.

Taliban militants often take over civilian homes and launch attacks on Afghan and coalition forces. U.S. officials say the militants hope to attract U.S. airstrikes that kill civilians, thereby giving the Taliban a propaganda victory.

After a massive case of civilian casualties in the village of Azizabad last August, McKiernan ordered forces to consider backing off from a fight if commanders thought civilians were in danger. Afghan officials and the U.N. say 90 civilians died in Azizabad; the U.S. says 33 died.

Meanwhile, NATO forces confirmed that two Afghan civilians were killed last week as international and Afghan forces battled insurgents in southern Helmand province. The troops called in an airstrike, which killed the two civilians and wounded three others.

In the capital, international and Afghan forces captured an insurgent financier who had provided funds for bomb-making materials, NATO forces said in a statement. He was arrested from a compound in Kabul without resistance, the statement said.