Reporting from Kabul, Afghanistan -- Afghan officials said Monday that at least 27 civilians were killed in a NATO airstrike in a southern province that borders Helmand, where thousands of coalition troops are taking part in an offensive in and around the town of Marja.

Western military officials said the incident in Uruzgan province, which occurred Sunday, was under investigation. It said U.S. Army Gen. Stanley A. McChrystal, the commander of all Western forces in Afghanistan, had already conveyed “sorrow and regret” to President Hamid Karzai, a highly vocal critic of civilian casualties.

Military officials said the airstrike was not connected to the assault on Marja, which is the largest coalition offensive since the start of the war in 2001. But it is certain to inflame sensitivities over the issue of civilian deaths and injuries in fighting between Western forces and the Taliban.

The incident apparently revolved around a trio of civilian minibuses being mistaken for an insurgent convoy as they traveled along a major highway. A statement from NATO’s International Security Assistance Force confirmed that “airborne weapons” were used against the vehicles, which at the time were believed to be carrying militants on their way to stage an attack.

But arriving ground forces found women and children at the scene, the military said. It did not say whether any suspected fighters were present as well.

NATO did not give an estimate of the number of dead, but Afghanistan’s Cabinet said at least 27 had died in the strike.

A spokesman for the Interior Ministry, Zemari Bashary, said more than a dozen other people were hurt, and NATO said the military had provided emergency treatment and transported them to medical facilities.

The Cabinet condemned the strike, calling it “unjustifiable.”

The incident threatened to overshadow what coalition forces had billed as an important milestone in Marja -- the first visit by the newly appointed civilian governor of the subdistrict. His arrival represented the first Afghan government presence in at least two years in Marja, which had become the largest Taliban sanctuary in Helmand province.

Insurgents had operated freely in the town and its environs until thousands of coalition troops, spearheaded by U.S. Marines, launched their effort Feb. 13 to retake the town.

In advance of the Marja campaign, Western commanders promised to take all possible measures to safeguard civilian lives. Marines had been fighting under stringent new rules of engagement laid down last summer by McChrystal. Troops are not supposed to fire on insurgents unless they are wielding a weapon, or unless they are seen throwing one aside.

Even so, NATO says at least 16 civilians have been accidentally killed by Western troops in the course of the offensive.

After days of intense clashes, the town was quieter on Monday, according to Marine spokesman Capt. Abraham Sipe.

“I don’t know if it’s a trend, but it’s good to see our Marines get a break after so much firefighting,” he said.

Staff writer Tony Perry contributed to this report from Nawa, in Helmand province.

laura.king@latimes.com