By Mirwais Harooni

KABUL (Reuters) - Afghan President Hamid Karzai on Friday accused the United States of failing to fully cooperate with an investigation into the massacre of 16 Afghan villagers by a U.S. staff sergeant and questioned whether only one soldier could have been involved.

A series of blunders by the United States, including the killings in Kandahar province on Sunday and the inadvertent burning of copies of the Koran at a NATO base last month, has further strained already tense relations between the countries.

"The army chief has just reported that the Afghan investigation team did not receive the cooperation that they expected from the United States. Therefore these are all questions that we'll be raising, and raising very loudly, and raising very clearly," Karzai told reporters at his palace.

Flanked by senior officials, Karzai met with village elders and the families of victims of the massacre, and dressed somberly in black for the start of two days of talks to discuss the killings.

Karzai appeared to back the belief of the villagers that one gunman acting alone could not have killed so many people, and in different locations some distance apart.

"They believe it's not possible for one person to do that. In his family in four rooms people were killed, women and children were killed, and they were all brought together in one room and then put on fire. That, one man cannot do," Karzai said.

With several investigations still underway by both U.S. and Afghan officials, any discovery of more than one soldier involved in the incident would be a disaster for NATO, with Western leaders needing to win over Afghans ahead of a withdrawal by most foreign combat troops in 2014.

On Thursday, Karzai called for NATO troops to leave Afghan villages and confine themselves to major bases, underscoring fury over the massacre and clouding U.S. exit plans.

He also demanded the handover of security to Afghan police and soldiers by 2013, a year ahead of schedule.

Such a move could undercut U.S. President Barack Obama's strategy for Afghanistan and hamper efforts to mentor Afghan police and help with local governance.

The soldier accused of carrying out the shooting was attached to a small special forces compound similar to others around the country which underpin NATO's anti-insurgent strategy.

Karzai has slim room for maneuver in a war that has tested both his presidency and that of his U.S. counterpart, Obama.

Installed by the United States as interim leader after U.S.-backed Afghan forces toppled the Taliban regime in 2001, he has to walk a line between growing public anger over the presence of Western troops and Washington's support of his government.

In a blow to NATO hopes of a negotiated end to the decade-old war, the Afghan Taliban said they were suspending nascent peace talks with the United States, following the massacre.

The U.S. government said it remained committed to political reconciliation involving talks with the Taliban but progress would require agreement between the Afghan government and the insurgents.

(Writing by Jack Kimball, Editing by Rob Taylor and Sanjeev Miglani)