Two US marines have been killed in a battle in eastern Afghanistan in which up to 23 militants are also thought to have died, the US military has said.
The US leads about 18,000 foreign soldiers in Afghanistan
The marines engaged the band of militants in Laghman province, east of Kabul, on Sunday, the Associated Press reported the military as saying.
There has been an upsurge in attacks on US-led forces following a winter lull.
About 18,000 US-led foreign troops are in Afghanistan tracking al-Qaeda and Taleban militants.
The US military said its warplanes were called in during the fighting in Laghman, an opium-producing region that has seen a number of clashes between US-led coalition forces and militants.
Two militants were confirmed dead and another 21 believed killed, the US military said.
The marines came under attack from small arms fire and rocket-propelled grenades launched by about 25 insurgents.
The two marines were killed while clearing a cave area.
Their names have been withheld until relatives are informed.
There has been no comment from Taleban sources or local Afghan officials yet on the fighting.
The Pakistan-based Afghan Islamic Press quoted a witness in Laghman as saying he had seen 17 people dead as a result of American bombing.
Nearly 150 American soldiers have died in Afghanistan since the US began its Operation Enduring Freedom in 2001.
Rebel attacks on US-led forces have risen in recent weeks, following the winter lull.
Seventy people died in three days of clashes in southern Afghanistan last week, Afghan and US authorities said.
The BBC's Andrew North in Kabul says the arrival of the spring thaw in the mountain areas where the Taleban-led insurgency has been concentrated, has always marked a new round of fighting in the past few years.
He says there had been hopes this year would be more peaceful amid statements from the US military that the Taleban and other groups were in disarray and that many wanted to give up.
However, the government is seen by some as having been slow in setting out plans for a reconciliation scheme with the Taleban, our correspondent says.
On Monday, the head of the country's recently established reconciliation commission said former Taleban leader Mullah Mohammed Omar and rebel warlord Gulbuddin Hekmatyar should be included in an amnesty.
Afghan President Hamid Karzai has previously said top militants should be barred.