Rockets fired at militants attacking NATO and Afghan troops missed and killed 12 civilians during an offensive in the southern Helmand province targeting a Taliban stronghold, NATO forces in Afghanistan said today.

‘It's regrettable that in the course of our joint efforts, innocent lives were lost. We extend our heartfelt sympathies and will ensure we do all we can to avoid future incidents,’ US General Stanley McChrystal, the NATO commander, says in a statement.

Afghan President Hamid Karzai expressed sadness at the incident.

He had urged NATO to exercise caution during the offensive to prevent civilian casualties.

Earlier, NATO commanders said the start of its major offensive against a key Taliban stronghold in southern Afghanistan had been 'a success'.

Marjah - a town of 80,000 in the central Helmand River valley controlled for years by militants and drug traffickers - is the focus of the NATO operation.

A Taliban commander said that NATO had hyped up the strategic importance of Marjah 'in order to cover up their past defeats'.

Mullah Abdul Razaaq's comments were made in an interview posted on the group's website.

Today is the second day of the 15,000-troop NATO operation - the biggest since overthrowing the Taliban regime in 2001.

Operation Mushtarak ('Together') aims to clear the area of Taliban and re-establish Afghan sovereignty and civil services, Helmand Governor Mohammad Gulab Mangal said.

Senior officials and NATO commanders said they were satisfied with the operation's first day, with Mark Sedwill, NATO's senior civilian representative, saying it 'appears to be positive'.

Senior British military spokesman Major General Gordon Messenger told a briefing in London that they 'very pleased with how it has gone'.

'The key objective has been secured,' he said, explaining that the main aims for British troops were to secure the population centres and installations such as police stations in the Chah-e Anjir Triangle northeast of Marjah.

Marjah has long been a breeding ground for insurgents and lucrative opium poppy cultivation, which Western countries say funds the insurgency.

Mushtarak is the first major assault on a Taliban stronghold since Mr Obama announced in December that he was sending an additional 30,000 soldiers to Afghanistan in 2010.

It puts into practice the new US-led counter-insurgency strategy combining the military objective of eradicating the Taliban with the need to replace their brand of harsh control with civilian authority.

The US and NATO already have 113,000 troops in the country battling the insurgents.

NATO has pledged another 10,000, bringing the total to more than 150,000 by August.

Afghan President Hamid Karzai warned the troops to do everything possible to avoid harming civilians.