KABUL, Afghanistan — U.S. troops on today reinforced a remote military outpost after well-armed militants got inside and killed nine American soldiers in the deadliest assault on U.S. forces in Afghanistan in three years.

Sunday's assault has deepened doubts about the U.S. military's ability to contain Islamic militants. Attacks in Afghanistan are becoming more complex, intense and better coordinated than a year ago, U.S. officials say.

Militants with machine guns, rocket-propelled grenades and mortars attacked the remote base in the village of Wanat in the mountainous northeastern province of Kunar at about 4:30 a.m. Sunday, with insurgents firing from homes and a mosque.

It was a "concerted attempt" to overrun the small base near the Pakistan border that was built only about three days ago, said an official with NATO's International Security Assistance Force.

The official, who spoke on condition of anonymity because he wasn't authorized to release the information, estimated the attacking force was several hundred.

An unknown number of militants got inside the outpost, the reason the fighters were able to inflict such high casualties, said a second military official who also spoke on condition of anonymity.

After the breach, U.S. troops pushed back against the invading militants, and attack helicopters swooped in. The second official said more than 40 insurgents were killed in the fighting. Fifteen U.S. soldiers also were wounded.

The latest assault came at a time of rising violence in Afghanistan.

Monthly death tolls of U.S. and NATO troops in Afghanistan surpassed U.S. military deaths in Iraq in May and June. And last Monday, a suicide bomber attacked the Indian Embassy in Kabul, killing 58 people in the deadliest attack in the Afghan capital since 2001.

U.S. officials are considering drawing down additional forces from Iraq in coming months, in part because of less violence in Iraq and the need for additional U.S. troops in Afghanistan. U.S. officials have said they need at least three more brigades in Afghanistan — or more than 10,000 troops.

NATO confirmed nine of its soldiers had been killed and 15 wounded. A Western official said the nine dead were Americans, speaking on condition of anonymity because he was not authorized to release the troops' nationalities. Four Afghan soldiers also were wounded, NATO said.

The attack was the deadliest for U.S. troops in Afghanistan since June 2005, when 16 American troops were killed — also in Kunar province — when their helicopter was shot down by a rocket-propelled grenade.

The latest assault came at a time of rising violence in Afghanistan. Also on Sunday, a suicide bomber targeting a police patrol killed 24 people, including 19 civilians, while U.S. coalition and Afghan soldiers killed 40 militants elsewhere in the south.

More than 2,300 people — mostly militants — have died in insurgency-related violence this year, according to an Associated Press tally of official figures. Attacks in eastern Afghanistan are up 40 percent this year compared with last year.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, warned during a visit to Kabul last week that there are more foreign fighters, including al-Qaida members, in Pakistan's tribal areas, militants who cross the border and launch attacks against U.S. and Afghan troops.