The biggest military operation of US President Barack Obama's new Afghan surge will be a test not just for American troops, but also for the Afghan authorities expected to rush with them into the breach.

US Marines are planning a massive operation within days to take Marjah, a warren of canals that forms the last big Taliban enclave in the southern part of Helmand, in the first major show of force since Mr Obama ordered in 30,000 extra troops.

Washington hopes the operation will help decisively turn the momentum this year in a war that commanders accept has been going the enemy's way.

For Afghan authorities, there is an equally big challenge: to show they finally have the capability to move in and quickly establish a government presence in a place that has been out of their grip for years.

The district governor of Marjah, Haji Zair, who has been able to visit but cannot live in the Taliban-held town, says residents repeatedly beg him to bring foreign and Afghan troops.

"The people every day ask the government for help. The elders who I contact, educated people, influential people, every day they ask the government to come and make contact and remove the terrorists," Mr Zair told Reuters in a telephone interview.

"Large numbers of (Taliban) are in Marjah. The people of Marjah are under the control and pressure of the Taliban, they cannot so far have much contact with the government," he said.

People in Marjah have been informed of plans to secure their town, Mr Zair said, through air-dropped leaflets, radio broadcasts and meetings with tribal elders.

Brigadier General Mohayedin Ghori, commander of an Afghan brigade that will participate in the operation, said Marjah has no police - unless some insurgents have dressed up in uniform.

"Anyone in uniform you know of, he is definitely an enemy," he added.

The town, a network of desert irrigation canals constructed decades ago under a US-funded development plan, is one of the few places left in Afghanistan's most dangerous province where the Taliban administers a shadow government and controls a comparatively large population of 75,000 to 100,000 people.

Under the new NATO strategy of commander General Stanley McChrystal, the main goal of US and allied troops is to secure population centres so that the Afghan government can move in.

Although commanders planning the operation do not disclose the size of the forces that will be involved, taking Marjah will be one of NATO's biggest operations ever in Afghanistan, and by far the biggest for Afghan troops since the war started in 2001.

Success will not only be a test of NATO's strength but also of the credibility and effectiveness of the Afghan army, which is under pressure to expand and needs to be ready to take over security in Afghanistan so Western troops can withdraw.

"If what we're doing today does not involves Afghan forces tomorrow, what are you doing here? If everything we're doing here helps prepare them, that's the right way to go," said Brigadier General Larry Nicholson, commander of US Marines in southern Afghanistan.

The Marines have made little secret of their intention to take the town, which General Nicholson said should motivate some inside it to begin contemplating joining the government.

"Because of the inevitability of the operation ... people have decided that they want to talk," he said. "There's a lot of influential people ... they don't want to be on the wrong side of this operation when it flips."

"When the Afghan flag's flying over Marjah they don't want to be seen as Taliban sympathisers or pariahs, they still want to be seen as people of import."

-Reuters