KABUL- A U.S.-led NATO force of thousands of troops began a long-awaited assault on the last big Taliban stronghold in Afghanistan's violent Helmand province on Saturday.

Here are some questions and answers about what has been billed as one of the largest offensives in eight years of war:

WHAT IS THE PRIMARY OBJECTIVE?

To break the Taliban's grip on Marjah, a district at the center of southern Helmand province's poppy production and the militants' biggest stronghold in the region. When it is secured, NATO intends to hand over the area to Afghan forces to show that they can control it and begin providing services to its roughly 100,000 residents. The assault is the first since U.S. President Barack Obama announced a surge of 30,000 troops to pacify Afghanistan, in anticipation of a troop withdrawal in 2011 as Afghan forces take over.

WHO IS TAKING PART?

The assault is led by the U.S. Marines, with British and Afghan troops also taking part in an overall force of about 7,500. The Marines are assaulting the area around Marjah, and the British are responsible for Nad Ali district to its east.

HOW MANY TALIBAN FIGHTERS ARE THERE?

No one knows, but estimates have ranged from a few hundred to several thousand. The U.S. military has said that some have already fled. One Taliban commander on Friday said some 2,000 militants were ready to fight.

WHY HAS NATO NOT KEPT THE OPERATION SECRET?

The assault has been flagged for months to persuade militants to leave the area without digging in for a fight that could lead to their mass destruction, on the scale seen in Iraq's Fallujah in 2004. NATO is also keen to avoid civilian casualties and has advised civilians not to leave their homes. Some have fled, but many have stayed.

WHAT ARE THE DANGERS?

Before the battle, U.S. commanders said they expected troops to face one of the largest-ever concentrations of improvised explosive devices (IEDs) and other booby traps as they advance.

The wider risk would be a prolonged fight or one that causes major civilian casualties or damage. That would undermine the important goal of putting Afghan forces back in charge, because it would give residents a reason to distrust or despise the U.S.-backed government.

(For more Reuters coverage of Afghanistan and Pakistan, see: here) (Editing by Michael Georgy and Tim Pearce)

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