Kabul, Afghanistan (CNN) -- The head of U.S. forces in Afghanistan, General Stanley McChrystal, expressed regret Monday after more than two dozen civilians were killed in a NATO airstrike.

Twenty-seven civilians died and 14 others were wounded in the incident Sunday in the central Daikondi province, according to the Afghan Interior Ministry.

Ground forces at the scene found women and children among the casualties, the Afghan government and NATO's International Security Assistance Force (ISAF) said in a joint statement.

The U.S military told CNN Pentagon Correspondent Barbara Starr they "accept" the Afghan government's death toll. ISAF said it had ordered an immediate investigation into the incident, while the Afghan cabinet called the attack "unjustifiable."

"We are extremely saddened by the tragic loss of innocent lives," said McChrystal, who spoke to President Hamid Karzai Sunday evening to express his sorrow and regret over the incident.

"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 re-double our efforts to regain that trust."

Commanders ordered the daylight NATO airstrike because they had specific intelligence that a group of Taliban in vehicles was heading towards coalition forces on the ground, according to a senior U.S. military official.

"Air assets picked up the movement of the vehicles and after an extensive overhead monitoring, the ground force commander ordered the strike," said the official, who declined to be identified because of ongoing investigations.

The source would not discuss what activities the convoy took that led to suspicions it contained insurgents other than its location.

The convoy of three vehicles was traveling to Kandahar province when it was struck, said Zemeri Bashary, the spokesman for the interior ministry.

NATO confirmed its forces fired on the vehicles, believing that they were carrying insurgents.

In a statement published in Pashtun and Dari, the Afghan cabinet said it condemned "the repeated killing of civilians by NATO." An English version of the statement did not include that sentence.

Civilian casualties at the hands of U.S. and NATO troops have strained relations between Afghanistan and the United States.

In the last two weeks alone, more than 50 Afghan civilians are believed to have been killed in more than half a dozen U.S. and NATO military operations.

The coalition is also investigating reports that several Afghan policemen were accidentally killed in an airstrike in eastern Afghanistan on February 18.

McChrystal has made avoiding civilian casualties a top priority, and he has apologized to the Afghan government for recent incidents.

The numbers have dropped in recent months since McChrystal took over as U.S. commander.

The U.S. military official said McChrystal is updating a directive issued to troops last summer aimed at reducing civilian casualties.

The official said the intent now is to "make it more precise and understandable by the most junior member of the force."

Some forces have complained the directive has led to overly restrictive rules on conducting operations.

Meanwhile, an influential Afghan tribal leader was among 14 people killed in a suicide attack on a meeting of tribal elders on Monday in Nangarhar province's Khogyani district, Afghan police said according to Agence France-Presse.

Haji Zaman Ghamsharik was credited with bringing relative stability to Nangarhar compared to other provinces in Afghanistan's volatile eastern border region.

Elsewhere, an insurgent rocket struck a car in Kapisa province Monday and killed one civilian, officials said. Five others were wounded in the attack.

Earlier, Afghan officials had blamed the attack on a NATO ground-to-ground missile but later corrected the account.

CNN's Ben Wedeman contributed to this report