LASHKAR GAH, Afghanistan--Taliban militants are digging in ahead of a major NATO operation in Helmand province in southern Afghanistan, in one of the biggest offensives in the eight-year-old war.

U.S. Marines are set to launch an operation within days to take Marjah, an area of lush farmland criss-crossed by canals in the centre of Helmand, Afghanistan's most violent province. The offensive will be the first major show of force since President Barack Obama ordered in 30,000 extra troops.

The operation has been flagged in advance in the hope militants will give up the fight in what commanders say is the last big Taliban enclave in the province. "It has to do with letting people know what's coming in the hope that the hardcore Taliban, or a lot of the Taliban, will simply leave, and maybe there will be less of a fight," U.S. Defense Secretary Robert Gates said in Turkey on Saturday.

But some of the villagers escaping Marjah in fear of their lives said fighters are digging in rather than fleeing. "The Taliban are not going to leave Marjah. We have seen them preparing themselves. They are bringing in people and weapons. We know there is going to be a big fight," said Abdul Manan, a man from Marjah who had fled to Helmand's capital, Lashkar Gah.

"The Taliban are very active in Marjah. They are planting mines there and in the surrounding areas," said villager Abdul Khaleq after arriving in Lashkar Gah with his family.

The United States and its allies, facing dwindling public support for the war, are hoping a big military push will convince the Taliban to accept a peace deal. But U.S. special envoy Richard Holbrooke dismissed speculation Washington, which wants to start drawing down troops in 2011, was already holding talks with the Taliban.

"I want to state very clearly that our nation is not involved in any direct contacts with the Taliban," he told reporters at a security conference in Munich, Germany.

Holbrooke said in principle negotiations and military operations could run in parallel, citing as examples the efforts to end the Vietnam war and the conflict in former Yugoslavia. "But it must go hand in hand with security success. It is not an alternative to the military campaign. It requires military success to make progress."

The Taliban have stepped up their fight against foreign troops in recent years, although they have largely shied away from face-to-face combat, relying instead on homemade bombs. But Abdullah Nasrat, a Taliban commander in Nad Ali district where Marjah is located, told Reuters by telephone there were some 2,000 insurgents there ready to fight to the death.