(Refiles to fix literal in first paragraph) (For more coverage of Afghanistan and Pakistan, see [nAFPAK])

By Peter Graff

KABUL, Feb 9 (Reuters) - Afghan villagers should stay inside and "keep their heads down" when thousands of U.S. Marines launch a massive assault on a densely-populated district in coming days, NATO's civilian representative to Afghanistan said on Tuesday.

U.S.-led NATO forces are planning one of the 8-year-old war's biggest offensives to seize Marjah, a patchwork of desert canals and opium fields that is now the last large Taliban-held bastion in Helmand, Afghanistan's most violent province.

The assault, the first since U.S. President Barack Obama ordered 30,000 extra troops to Afghanistan in December, is the start of a campaign to impose government control on rebel-held areas this year, before U.S. forces start to draw down in 2011.

Western countries hope military success this year will persuade Taliban fighters to lay down arms and their leaders to accept invitations to talk.

Hundreds of civilians have fled, but most of the area's population, estimated at up to 100,000, remain in their homes in the face of what could be an unprecedented level of fighting.

NATO civilian representative Mark Sedwill said "sufficient" plans were in place to feed and house any civilians who flee, but declined to give details of how many displaced people NATO or the Afghan authorities had the capacity to assist.

"There are good reasons not to go into the exact numbers of details of the operation at this stage," Sedwill, a former British ambassador who arrived this week to take up the beefed-up post of chief NATO civilian in Kabul, told a news briefing.

"What we can say is that we are confident there are sufficient resources there to accommodate and feed anybody who chooses to leave the area," he said.

Helmand's governor, Gulab Mangal, said so far about 164 families have fled the area in recent days. A commission had been set up that has the capability to cope with any crisis, he said.

NATO forces have decided to advise civilians in Marjah not to leave their homes, although they say they do not know whether the assault will lead to heavy fighting.

HEADS DOWN, STAY INSIDE

"The message to the people of the area is of course, keep your heads down, stay inside when the operation is going ahead," Sedwill said.

"We very much hope that the military phase of this operation will go ahead swiftly and with as little incident as possible. This of course very much depends on the conduct of those people who are in Marjah at the moment, their choices about whether to resist or to lay down their weapons."

Unlike previous military operations, the assault on Marjah has been widely flagged for months. Commanders say they hope this will persuade many fighters to lay down their arms or flee, reducing the eventual death toll.

Civilians who have left the area, however, report that fighters are digging in and preparing for battle.

Under international law, NATO forces are obliged to provide humanitarian assistance to anyone who chooses to flee the assault, said Brad Adams of Human Rights Watch.

Having advised civilians to stay instead -- helping ensure the area remains heavily populated during the offensive -- they bear an extra responsibility to control their fire and avoid tactics that endanger civilians.

"I suspect that they believe they have the ability to generally distinguish between combatants and civilians. I would call that into question, given their long history of mistakes, particularly when using air power," Adams said.

"Whatever they do, they have an obligation to protect civilians and make adequate provision to alleviate any crisis that arises," he said. "It is very much their responsibility.... They are going to be carrying the can if this goes badly." (Editing by Alex Richardson) (For more Reuters coverage of Afghanistan and Pakistan, see: http://www.reuters.com/news/globalcoverage/afghanistanpakistan)

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