KABUL – NATO jets mistakenly killed at least 21 people in central Afghanistan, Afghan officials said Monday, in the deadliest attack on civilians in six months. The strike prompted a sharp rebuke from the Afghan government as it struggles to win public backing for a major military offensive against the Taliban in the south.

The civilian deaths occurred as 15,000 NATO, U.S. and Afghan soldiers were in their 10th day of fighting insurgents in the southern town of Marjah in Helmand province. The mission is to rout the Taliban, set up a local government and rush in aid to win public support.

The alliance said its planes fired on what was thought to be a group of insurgents in Uruzgan province on their way to attack NATO and Afghan forces. Interior Ministry spokesman Zemeri Bashary said the airstrike hit three minibuses, which were traveling on a major road near Uruzgan’s border with Day Kundi province.

Although the airstrike was not related to the Marjah offensive, civilian casualties undermine NATO’s goal of turning back the Taliban and winning the confidence of the Afghan people, one of the main objectives of the southern operation.

Afghan President Hamid Karzai has repeatedly called on NATO to do more to protect civilians during stepped-up military operations, and the Afghan Cabinet strongly condemned the airstrike.

In recent months, NATO has limited airstrikes and tightened rules of engagement on the battlefield to try to protect the Afghan people and win their loyalty from the Taliban.

”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,” top NATO commander Gen. Stanley McChrystal said in a written apology after Sunday’s strike. ”We will redouble our effort to regain that trust.”

It was the second time in nine days that NATO has apologized for killing civilians. On Feb. 14, two U.S. rockets slammed into a home outside Marjah, killing 12 people, including six children.

According to NATO, at least 16 civilians have been killed so far during the offensive; human rights groups say the figure is at least 19.