With 28 months left in the current mission, American policymakers are struggling with “green on blue” attacks.
The Taliban have claimed responsibility for a deadly attack on the military post in southern Afghanistan where Britain’s Prince Harry is based.
Friday’s attack at Camp Bastion in Helmand province left two US soldiers dead and some others wounded, US officials said.
The Taliban said it was in response to the anti-Islamic video that has sparked a wave of ongoing protests, Al Jazeera’s Afghanistan correspondent Bernard Smith reported from Kabul.
Camp Bastion is a British airbase and is adjacent to Camp Leatherneck, the main base for the US Marine Corps in Helmand.
A defence official in Washington said the two dead were US marines, speaking on condition of anonymity, while another US official described the attack as “complex”, meaning it was a co-ordinated strike using several types of weapons.
The attack, involving small arms and mortar or rocket fire, started around midnight local time, Master Sergeant Bob Barko of NATO’s International Security Assistance Force (ISAF) told AFP news agency.
Another ISAF spokesman said the assault was over but details including the number of attackers and whether they managed to penetrate the base were not immediately clear.
Prince Harry, 27, has been deployed at Camp Bastion as a military helicopter pilot.
According to Smith, the Taliban also claimed that the prince was a target in Friday’s attack.
Taliban fighters have threatened to kill him, saying earlier this week they had a “high-value plan” to attack the third in line to the British throne.
Barko said that ISAF was assessing the extent of the damage to the camp in Helmand, one of the toughest battlefields in the war, but Harry was not thought to have been affected.
“The information we have is that he was not in any danger,” he said.
Harry will spend four months based at the heavily fortified Camp Bastion.
In 2008, he was hastily withdrawn from Afghanistan when a news blackout surrounding his deployment, on the ground directing aircraft in attacks on Taliban positions, was broken.
This time, however, the military has released photographs and video of him in Afghanistan from the start.
Britain’s defence ministry said any risk “has been, and will continue to be, assessed”.
NATO has about 117,000 troops in Afghanistan fighting the Taliban alongside government forces.
Most of the NATO troops are set to withdraw by the end of 2014 in a US-designed transition process that will put Afghan security forces in charge of security for their war-battered country.
The process is already under way with security responsibilities of about half of the Afghan population transferred to the local security forces.
The Taliban have stepped up their attacks in recent months as part of efforts by the group to undermine the transition process.
Helmand, in the troubled south, was the focus of a 30,000-strong troop surge announced by the US in 2009 designed to quell the Taliban-led fight once and for all.
A total of 327 international soldiers have been killed in Afghanistan so far this year, according to the iCasualties website, 250 of them American.
The toll does not include those who died in the latest attack.