NATO commanders apologized Monday after a coalition airstrike killed at least 27 Afghan civilians, including women and children. Monday's strike was the third this month to kill noncombatants and drew a sharp rebuke from Afghanistan's government about endangering civilians.

In eastern Afghanistan, a homicide bomber killed 15 people on Monday, including a tribal leader who played a key role in a failed attempt to capture Al Qaeda leader Usama bin Laden at Tora Bora in 2001, police said.

The top NATO commander, U.S. Gen. Stanley McChrystal, apologized to President Hamid Karzai for the Sunday airstrike, which occurred in the central Afghan province of Uruzgan.

The Afghanistan Council of Ministers strongly condemned the airstrike, calling it "unjustifiable."

It said reports indicated that NATO planes fired at a convoy of three vehicles, killing at least 27 people, including four women and a child, and injuring 12 others.

The ministers urged NATO to "closely coordinate and exercise maximum care before conducting any military operation" to avoid further civilian casualties.

NATO confirmed that its planes fired on what it believed was a group of insurgents on their way to attack NATO and Afghan forces, but later discovered that women and children were in the vehicles. A number of people were killed and the injured were transported to medical facilities, NATO said in a statement.

Interior Ministry spokesman Zemeri Bashary said the airstrike hit three minibuses traveling on a major road near Uruzgan's border with Day Kundi province. There were 42 people in the vehicles, all civilians, he said.

Bashary said local investigators had collected 21 bodies and two people were missing. He said he was checking with Cabinet officials to find out why there was a discrepancy in the toll.

"We are extremely saddened by the tragic loss of innocent lives," Gen. McChrystal said in the NATO statement. "I have made it clear to our forces that we are here to protect the Afghan people and inadvertently killing or injuring civilians undermines their trust and confidence in our mission. We will redouble our effort to regain that trust."

The attack was not related to the ongoing offensive around the Helmand province town of Marjah, where U.S. and Afghan forces have been battling Taliban insurgents since Feb. 13.

At least 16 civilians have been killed so far during the offensive, NATO said, though human rights groups say the number is at least 19. They include nearly a dozen people killed when two NATO rockets struck a house on the outskirts of Marjah on the second day of the offensive.

Last Thursday, an airstrike in northern Kunduz province missed targeted insurgents and killed seven policemen.

The Sunday attack was the worst involving civilians since last September, when U.S. pilots bombed two hijacked fuel tankers in a German-ordered airstrike near the northern town of Kunduz. German officials, citing a classified NATO report, say up to 142 people are believed to have died or been injured. Afghan leaders estimate 30-40 civilians were killed.

Police Gen. Mohammad Ayub Salangi said Monday's suicide attack occurred in Khogyani district, outside of Nangarhar province's capital, Jalalabad, when a group of tribal elders and government workers were meeting with a few hundred Afghan refugees who had recently returned from Pakistan.

Among those killed was tribal leader and former mujahedeen fighter Mohammad Zaman Ghamsharik, better known as Haji Zaman. He and another warlord from the Jalalabad area, Hazrat Ali, commanded Afghan forces who cornered bin Laden in the mountains of Nangarhar province but failed to keep him from slipping away.

Last Saturday, Karzai called on NATO to do more to protect civilians during stepped-up military operations against the Taliban.

NATO has taken steps in recent months to reduce civilian casualties — primarily through reducing airstrikes and tightening rules of engagement — as part of a new strategy to focus on protecting the Afghan people to win their loyalty over from the Taliban.

A total of 2,412 Afghan civilians were killed last year, the highest number in any year of the eight-year war, according to a U.N. report. But deaths attributed to NATO troops dropped nearly 30 percent as a result of new rules curbing airpower and heavy weapons when civilians are at risk, it said.

That strategy is facing a major test in Marjah, where Taliban fighters have mingled among civilians in hopes that U.S. and Afghan troops will hold their fire.

Fighting in Marjah was less intense Monday than in previous days, said Marine spokesman Capt. Abe Sipe.

"There were fewer engagements between insurgents and the Marines and Afghan national security forces in town," he said.

Sipe said NATO forces are encouraged that more residents are reopening shops and that empty compounds are filling up with families that have returned.

New district leader Abdul Zahir Aryan flew into Marjah for the first time and held a shura, or meeting, with about 50 community elders, Sipe said.

The International Committee of the Red Cross said Monday that it had evacuated 28 sick and injured civilians from Marjah to treatment facilities outside the area since the beginning of the offensive. Most required lifesaving medical treatment, it said.

Meanwhile, in the east of the country, NATO forces said they killed 14 insurgents in two separate clashes — one in Kapisa province and one in Kunar.

Fox News' Conor Powell and The Associated Press contributed to this report.