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.

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.