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Just before dawn today, the still-darkened skies over a besieged Afghan town were filled with more than 50 helicopters, including a flight of Canadian aircraft, launching an assault on the Taliban-held town of Marjah, in southern Helmand province.

Three Canadian CH147 Chinook helicopters, covered by the guns of four tactical CH146 Griffons, were part of the massive air assault which landed hundreds of U.S. Marines, NATO and Afghan troops near Taliban lines for a long-anticipated attack on the town.

The assault launched Operation Moshtarak -- which means "together" in Dari -- the biggest campaign yet in the nine-year war in Afghanistan and a key test of U.S. President Barack Obama's revamped strategy for the southwest Asian country.

The air attack was also the biggest mission ever for Canada's air wing, based in neighbouring Kandahar province.

"This is the big show for us. This is the ultimate kind of goal for us," Helicopter wing commander Col. Jeff Smyth told CTV's Janis Mackey Frayer from the Canadian wing's base at Kandahar Air Field.

The offensive is designed to seize the town of Marjah and clusters of nearby villages in central Helmand province, an area ruled by drug lords and insurgents who intimidate locals.

Col. Richard Leakey, the British army commander of the operation, said his intent is to drive out the Taliban and allow the ordinary Afghans to get on with their lives without fear of intimidation by the insurgents.

"They need to be able to go about their normal lives," he said. "We need to allow them to do that with a sense of security."

The more than 15,000 coalition soldiers involved in the operation were expecting to face bitter opposition from the Taliban. Much of the operation will be carried out by foot soldiers advancing into defensive lines laced with land mines and bombs. Taliban fighters, dug into trenches and well-fortified defensive positions have already fired mortars into coalition positions and vowed to mount hit-and-run counter-attacks to throw the attackers off balance.

Among the infantry attacking Marjah will be about 30 Canadians, trainers and mentors attached to an Afghan National Army battalion participating in the operation.

Operation Moshtarak is seen as a crucial measure for the competence of Afghan forces.

"Our goal is to have them operate independently," said Col. Shane Brennan, commander of the Canadian troops training the Afghans.

Hundreds of Afghan villagers jammed the main road out of the besieged town on Friday ahead of the attack, defying the Islamist insurgents' orders and fleeing Marjah.

Thousands of U.S., British and Afghan troops had encircled Marjah, a major insurgent supply base and opium-poppy centre, 610 kilometres southwest of Kabul, and spent days preparing their attack to clear Taliban fighters from the farming community and restore government control.

Alain Pellerin, a retired Canadian army colonel and executive director of the Conference of Defence Associations, told Â鶹´«Ã½ Channel that the mission was the largest ever undertaken by NATO forces in Afghanistan.

"It's no doubt the largest military operation in Afghanistan," he said.

The Marjah operation began weeks ago and coalition commanders signalled their intention to attack Marjah well in advance in hopes that civilians would seek shelter.

"This must be the most publicized attack in Afghanistan that we've seen thus far," Pellerin said.

But instead, residents told The Associated Press by telephone this week that Taliban fighters were preventing them from leaving, warning the roads were planted with land mines to slow the NATO soldiers' advance.

Many of the people who filled the hundreds of cars and trucks jamming the road between Marjah and the provincial capital of Lashkar Gah, 30 kilometres to the northeast, said they had to slip out of town when Taliban commanders weren't watching.

"We were not allowed to come here. We haven't brought any of our belongings; we just tried to get ourselves out," said Bibi Gul, an elderly woman in a black headscarf who arrived in nearby Lashkar Gah with three of her sons. She left three more sons behind in Marjah.

Afghan police searched the vehicles for any signs of militants, in one case prodding bales of cotton with a metal rod in search of hidden weapons.

"They don't allow families to leave," Marjah resident Qari Mohammad Nabi said of the Taliban. "The families can only leave the village when they are not seen leaving."

Pellerin said the British, U.S. and Afghan forces appear to have surrounded 1,000 or more Taliban fighters in Marjah.

"It seems that more and more it's becoming a trap for them," Pellerin said. "This is a fairly big town, between 80,000 and a 100,000 people live in Marjah. So it looks like we might be facing a battle."

And despite the mass exodus Friday, Pellerin warned that there are likely still enough people left in Marjah to allow the Taliban to use them as human shields.

"A lot of the civilian population have left the town already in anticipation of the battle, but there are still some that stayed behind," he said. "So our forces will have to be very conscious of the civilian population that remained behind and try to avoid them. But the Taliban could very well use them as a shield, as they have done in the past."

Pellerin said he expects the attack on Marjah to be a slow and deliberate advance, with the first wave of soldiers looking for land mines, booby traps and improvised explosive devices (IEDs) planted by the insurgents to stall the coalition's progress.

"It will be a very careful operation. The Taliban have had a long time to plant all the IEDs and bombs that they wanted to plant."

Tribal elders have pleaded for NATO to finish the operation quickly and spare civilians -- an appeal that offers some hope the townspeople will co-operate with Afghan and international forces once the Taliban are gone.

Once the town is secured, NATO hopes to rush in aid and restore public services in a bid to win support among the estimated 125,000 people who live in Marjah and surrounding villages.