U.S. soldiers train Sunday in Helmand province, southern Afghanistan, where NATO and Afghan forces are expected to launch an offensive soon. (Pier Paolo Cito/Associated Press)

Thousands of residents near the Afghan town of Marja are fleeing their homes in anticipation of an expected NATO offensive against the Taliban stronghold.

American and British troops working alongside Afghan forces are expected to launch a major assault soon on the town in southern Helmand province in what could be one of the largest counter-insurgency operations since the conflict began in 2001.

Operation Moshtarak — "Together" in the Pashtun language — is expected to begin within the next few days. It is seen as a major test for Afghan forces, who NATO countries want to see take over more responsibility in anticipation of an eventual pullout of allied forces.

Afghan President Hamid Karzai has told his NATO allies that he wants to build and train an army and police force of 300,000 by 2012, aiming to provide security for Afghanistan by 2015 without foreign help.

NATO aircraft dropped leaflets over Marja on the weekend warning of the planned offensive and urging residents and fighters to leave the area. Provincial officials said as many as 35,000 people were heeding the warning and fleeing, but the International Committee of the Red Cross said there was no way to confirm the number.

Time of offensive 'a closely guarded secret'

British Brig.-Gen. James Cowen said though the plan to launch the operation is known, NATO forces would still have an advantage because they control when it will occur.

"The critical piece of information is actually the precise timing," said Cowen. "That is a very closely guarded secret, and so the enemy do not have that advantage."

"I don't think we've lost anything by doing this and I think we will find the enemy will either choose to run away or to reintegrate, and only those we wish to kill will choose to fight," he said.

Canadian helicopter crews have ferried British and Danish forces into Helmand in the last few weeks to help gather intelligence for the mission.

It is the first major operation since U.S. President Barack Obama announced 30,000 more American troops would join the fight in Afghanistan as the embattled Karzai government attempts to repulse insurgents aligned with the Taliban, the militant former rulers of the country who were driven from power in 2001.

Meanwhile, British defence officials said Monday that two British soldiers were killed Sunday by a roadside bomb in Helmand during a mission unrelated to the Marja operation.

With files from The Associated Press