Twelve Afghan civilians have been killed in a rocket attack by joint NATO and Afghan troops.

The incident came on the second day of the military offensive on the Taliban stronghold in Marjah. NATO has expressed regret for the incident.

NATO said two rockets from a High Mobility Artillery Rocket System were aimed at insurgents firing on Afghan and NATO forces. However the rocket struck 300 metres off the intended target.

General Stanley McChrystal, the top US commander in Afghanistan, said coalition forces "deeply regret" the incident.

U.S.-led NATO troops launched a crucial offensive on Saturday against the Taliban's last big stronghold in Afghanistan's most violent province and were quickly thrown into a firefight with the militants. [Xinhua]

Before the offensive began on Saturday, Afghan President Hamid Karzai asked Afghan and foreign military leaders to give high priority to the safety of civilians.

Mohammad Ebrahim, Villager, said, "They come here to fight and then they leave. Before, there was no security. Now things are safer, we want them to build us schools and roads. But so far they have done nothing for us."

Between four hundred and a thousand insurgents - including more than 100 foreign fighters - are believed to be holding out in Marjah.

The offensive, called "Moshtarak", or "Together", is the biggest joint operation since the 2001 US-led invasion of Afghanistan. 15-thousand troops are involved, including around 7,500 in Marjah itself.

NATO said that several shuras, or council meetings, had been held with residents in Nad Ali and Marjah. More meetings are planned, as part of a larger strategy to gain support for the mission from local communities.