A U.S. Army Black Hawk medical evacuation helicopter takes off at a forward operating base south of Marjah, in Helmand province, southern Afghanistan, on Friday. ((Brennan Linsley/Associated Press))

Canadian Chinook helicopters touched down Friday in a Taliban stronghold in Afghanistan's restive south as coalition forces mounted the largest air assault of the nine-year war.

American, British, Afghan and other coalition troops were storming the insurgent-held town of Marjah and the district of Nad Ali, said to be one of the last major bastions of Taliban control in Helmand province.

The attack is called Operation Moshtarak — meaning "together" in Dari — and it is by far the largest offensive staged since U.S. President Barack Obama's decision to send 30,000 more troops into Afghanistan to try to quell a spreading insurgency.

Three Canadian Chinook helicopters were helping ferry about 1,100 coalition troops to Nad Ali, under the watch of four Canadian Griffon escorts.

The seven Canadian choppers joined 33 British and American helicopters in the assault on Nad Ali. Before the attack, military officials said their intelligence showed between 150 to 200 insurgents hunkered down in small pockets around Nad Ali.

The militants were believed to be armed with machine-guns, rocket-propelled grenades, anti-aircraft weapons and other light weaponry.

Even more helicopters and troops attacked Marjah, southwest of the Helmand provincial capital of Lashkar Gah. Thirty Canadian military trainers were part of the Marjah assault with their Afghan army pupils.

Thousands of coalition troops were part of the dual attacks on Marjah and Nad Ali. About half the soldiers are Afghan.

For weeks now special forces have been targeting Taliban leaders and bomb-making factories in the area ahead of what could be one of the largest and bloodiest stands of the Afghan war.