WASHINGTON -- The United States has extended the stay of 2,200 marines in Afghanistan for one month until November, making their tours eight months long instead of seven, a marines spokeswoman said Thursday.

The extension had been requested by the NATO-led International Security Assistance Force (ISAF) and was approved by Secretary of Defense Robert Gates "within the week," Captain Amy Malugani said.

The change applies to members of the 24th Marine Expeditionary Unit, who are deployed in Helmand province along with Canadian, British and Dutch soldiers in the south where forces are fighting a resurgent Taliban.

The Pentagon had decided to send the unit, along with 1,000 Marines in charge of training Afghan security forces, in response to requests for reinforcement from NATO troops on the ground, in a country that last year saw its bloodiest year in six years of war.

However, the Defense Department had until now stressed that the reinforcements would be sent only for a limited period, and that their mission would terminate after seven months.

ISAF has around 50,000 soldiers from 40 nations, up from 33,000 troops 18 months ago. But military officers say they need more reinforcements to fend off the rising Islamist insurgency.

The month of June was the bloodiest for foreign soldiers since the start of the war that ousted the Taliban from power in 2001, following the September 11 attacks by Al-Qaeda militants on the United States.

Forty-nine soldiers from ISAF or the separate US-led coalition were killed either in combat or in accidents in June, according to an AFP count based on military figures.

That's more than in Iraq, where 31 soldiers, 29 of them Americans, were killed during the same time period according to icasualties.org.

President George W. Bush on Wednesday acknowledged "a tough month" for NATO and US-led forces in Afghanistan but insisted the war-fighting strategy there was working, despite increasing violence.

Admiral Michael Mullen, chairman of the Joints Chiefs of Staff, confessed meanwhile to being "deeply troubled" by recent military challenges in Afghanistan, as Taliban troops ramp up their attacks on Western targets.

"I am and have been for some time now, deeply troubled by the increasing violence there," Mullen said.

"The Taliban and their supporters have, without question, grown more effective and more aggressive in recent weeks, and as the casualty figures clearly demonstrate," he said.

Bush nevertheless remained upbeat.

"I'm confident that the strategy is going to work," Bush said, sidestepping a question on whether he would send more US troops to Afghanistan by saying that he is constantly reviewing the needs on the ground.