War planes were called in to assist troops on the ground in the six-hour sporadic firefight in the southern province of Zabul in which another 20 rebels were wounded, the International Security Assistance Force (ISAF) said in a statement.
ISAF does not release the nationalities of its casualties, leaving that to the soldiers' home nations.
Most of the troops in Zabul are US nationals, who make up around half of the more than 110 foreign soldiers killed in action in Afghanistan this year.
An ISAF patrol "decisively engaged and killed 55 insurgents and wounded 20 more earlier today in the Daychopan district of Zabul province," the statement said.
It was the second major battle in the area in as many days.
Afghan and ISAF troops killed up to 70 insurgents in a battle involving attack helicopters and air support in neighbouring Uruzgan province on Saturday.
An ISAF soldier was killed and eight wounded in a separate attack in Uruzgan on Saturday but the nationalities of the troops involved still have not been released. ISAF has based Australian and Dutch soldiers in the province.
Monday's operation "will help to defeat those who threaten security and the onset of further reconstruction and development in the region," the ISAF statement said.
The 37-nation force aims to stabilise Afghanistan so that its war-shattered infrastructure can be built up and the country can leave behind the lawlessness that allowed the extremist Taliban come to power in 2001.
The Taliban sheltered the Al-Qaeda terror network behind the September 11, 2001 attacks, which led the United States to invade Afghanistan when the regime did not hand over al Qaeda leaders.
The NATO alliance, in the most ambitious military operation in its history, has faced intense fighting here.
In another clash announced on Monday, ISAF said three of its soldiers were wounded on Sunday when they were attacked by rebels in the volatile Kunar province near the border with Pakistan.
A purported Taliban spokesman reportedly said that fighters with the extremist movement had carried out the attack.
The area is close to the region of Pakistan where military helicopter gunships destroyed on Monday an Islamic school used as an al Qaeda-linked training camp, killing up to 80 militants, officials and witnesses said.
The movement of militants across the border between Afghanistan and Pakistan is one of Kabul's main concerns, with officials saying the fighters are feeding the insurgency that is crippling the country.

Copyright AFP (Agence France-Presse), 2006