Kabul - Two roadside bombs killed six American troops in Afghanistan on Monday, as a suicide bomber attacked the gate of the main NATO base in the south, officials said.

    The attacks come as thousands of U.S. Marines continued with their massive anti-Taliban offensive in the south, the biggest U.S. military operation since the ouster of the Taliban from power in 2001.

    American troops recently received new guidelines limiting use of airstrikes in order to minimize Afghan civilian casualties that threaten local support of foreign forces' presence.

    Four U.S. soldiers died when their vehicles struck a roadside bomb in Kunduz province in the north, said Navy Chief Petty Officer Brian Naranjo, a U.S. military spokesman. The dead were training Afghan forces, Naranjo said.

    In comparison to the country's south and east, northern Afghanistan is relatively quiet. But roadside and other insurgent attacks have been increasing in the last few years, as the militant's step up their operations.

    In the south, meanwhile, another explosion killed two more American troops, Naranjo said, without providing details of the exact location of the blast.

    The six American deaths came as thousands of U.S. Marines continued with their major offensive against the Taliban in neighboring Helmand province , a militant stronghold and hub of the vast Afghan drugs trade.

    The Taliban have made a violent comeback in the last three years following their ouster from power in the U.S.-led invasion in 2001. The militants now have effective control of large chunks of the volatile south and east of the country, undermining Afghanistan's government.

    Some 500 Marines out of the group of 4,000 participating in the Helmand offensive have moved into the province's Khan Neshin area, a Marine statement said Monday.

    "This is the first time coalition forces have had a sustained presence so far south in the Helmand River valley. Khan Neshin had been a Taliban stronghold for several years before Afghan, and coalition forces arrived and began discussions with local leaders several days ago," it said.

    In an attempt to minimize civilian deaths in coalition military operations, the new commander of U.S. and NATO forces, Gen. Stanley McChrystal, issued new guidance to forces in Afghanistan, saying that he expects them to "scrutinize and limit" the use of airstrikes against residential compounds "and other locations likely to produce civilian casualties."

    McChrystal has said he hopes to produce a cultural shift in the military so that troops don't see the use of massive firepower as their first priority but rather the protection of Afghan civilians. McChrystal's new guidelines went into effect last week, and officials released a declassified version Monday.

    "We must avoid the trap of winning tactical victories - but suffering strategic defeats - by causing civilian casualties or excessive damage and thus alienating the people," the new order says.

    McChrystal said that the Taliban cannot defeat U.S. and NATO forces but that "we can defeat ourselves." The directive allows soldiers to fire on residential compounds when it's necessary for self defense.

    Civilian deaths caused by U.S. and NATO military operations have long been a source of friction between President Hamid Karzai and the West. Such deaths alienate Afghan villagers, causing a loss of support for the Afghan government and international mission.

    A NATO helicopter, meanwhile, made an emergency landing in the southern Zabul province, a spokesman for the military alliance said. There were casualties among those onboard but Lt. Commander Chris Hall did not have further details.

    The incident was not caused by insurgent fire, Hall said.

    --------

    Associated Press reporter Geir Moulson in Berlin and Jason Straziuso in Camp Leatherneck, Afghanistan contributed to this report.