Afghan soldiers, covered by U.S. marines, run across an open area in Marjah on Sunday to avoid fire from Taliban snipers. (David Guttenfelder/Associated Press)

The top NATO commander in Afghanistan apologized to Afghan President Hamid Karzai Sunday after two NATO rockets missed insurgents and killed 12 Afghan civilians.

"We deeply regret this tragic loss of life," Gen. Stanley McChrystal said in a news release.

The rockets missed the target — a compound where insurgents were firing at NATO and Afghan soldiers, wounding two — by 300 metres, the release said.

The civilians were killed in the Nad Ali district, one of the areas near Marjah in Helmand province where 15,000 NATO and Afghan troops began a major operation Friday to dislodge the Taliban.

Karzai had previously asked military leaders to avoid civilian casualties. On Sunday, he issued a statement saying 10 members of the same family were killed by a rocket.

The death of Afghan civilians during NATO operations has undermined the coalition's efforts to win over the population.

The Marjah attack, dubbed Operation Moshtarak ("together" in Dari, the language spoken in Afghanistan), could take up to 30 days to succeed, U.S. marine Brig.-Gen. Larry Nicholson said Sunday.

But he was "more than cautiously optimistic" it will be sooner. Marines and Afghan soldiers had occupied most of Marjah by Sunday, but sniper fire continued, forcing Nicholson to take cover at one point.

"The fire we just took reflects how I think this will go — small pockets of sporadic fighting by small groups of very mobile individuals," he said.

An Afghan official said at least 27 insurgents have been killed. A British and an American soldier have been killed.

Marines are a key part of the forces involved. Canada participated by contributing seven helicopters to airlift 1,100 British and Afghan soldiers to Nad Ali on Friday. A small group of 34 Canadian soldiers are also involved as mentors to Afghan troops.

With files from The Associated Press