U.S. Marines leading one of NATO's biggest offensives against the Taliban in the eight-year war are facing fierce resistance in some areas, including heavy gunfire, snipers and booby traps, U.S. officials said on Monday.

Marine units have tried twice since Sunday to reach a bazaar in Marjah, the last militant stronghold in the country's most violent province, Helmand, only to be pushed back.

Coming under heavy gunfire and sniper attacks -- with one assault lasting over an hour -- they were forced to call in Harrier jets and attack helicopters with Hellfire missiles.

There have been conflicting assessments of how much progress NATO has made but one thing seems clear: the campaign to seize insurgent-held areas ahead of a planned 2011 troop drawdown could drag on for weeks.

We are making steady progress, but being very methodical about detecting and clearing routes in an area heavily saturated with IEDs (Improvised Explosive Devices), Marine Capt. Abraham Sipe told Reuters in response to an e-mail, adding counts of militants killed of captured would not be provided.

In many parts of Marjah, we have seen very little opposition. There are areas where Marines have met with stiff resistance, but they are making steady progress throughout the area.

The assault is the first test of U.S. President Barack Obama's plan to send 30,000 more troops to Afghanistan, where the Taliban have made a steady comeback since a U.S.-led invasion ousted them in 2001.

The marines' efforts to come within striking distance of Taliban snipers in Marjah's bazaar came a day after the compound the U.S. troops are holding in the heart of the town was attacked several times, showing the Taliban are not confined to one area.

Afghanistan is a top foreign policy issue for Obama so failure here could be seen as damaging to his presidency.

Much of the operation's success in Helmand province depends on whether the administration wins residents' trust and Afghan troops are able to keep the Taliban from returning.

Afghan officials said on Sunday that as many as 35 militants had been killed in the first two days of the offensive.

In a statement on the group's website, Taliban spokesman Qari Mohammad Yousuf, citing reports from Helmand, said fighters pushed back attacks by NATO troops on Monday.

CIVILIAN ISSUE

NATO and the Afghan government's credibility rests on limiting civilian casualties, especially as NATO commanders told Marjah residents to stay home during the offensive.

NATO rockets killed 12 civilians on Sunday, the second day of a drive to impose Afghan authority on Marjah, an area of farmland criss-crossed by canals, a breeding ground for insurgents and lucrative opium poppy cultivation, which Western countries say funds the insurgency.

Highlighting the dangers of fighting a resilient and unpredictable enemy, Helmand Province Governor Gulab Mangal said three would-be suicide bombers were gunned down on Sunday while trying to blow themselves up among troops.

The situation moment by moment is going the way the government had expected. The forces are extending their advances from points they have captured and the operation is going on successfully, he told a news conference.

The Taliban could not be reached for comment on Monday.

But in a statement on the 20th anniversary of the withdrawal of defeated Soviet troops from Afghanistan, after battling Western-backed mujahideen fighters for nearly a decade, the Taliban said:

The current occupiers of Afghanistan, like the Red Army, will face defeat.

Twenty years after the defeat of the Red Army, today Obama, also in Afghanistan, has given one-and-a-half years to the commander of foreign invaders, (NATO Commander U.S. General Stanley) McChrystal, to prove his success against the Islamic Emirate.

The offensive has been flagged for weeks to persuade Taliban fighters to leave so the area can be recaptured with minimal damage or loss of civilian life, in the hope that the roughly 100,000 people there will welcome the Afghan administration.

The attack started on Saturday with waves of helicopters ferrying troops into Marjah and nearby Nad Ali district.

(Additional reporting by Sayed Salahuddin in KABUL; Writing by Michael Georgy; Editing by Bryson Hull and Raju Gopalakrishnan)

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