The International Security Assistance Force (ISAF) announced on Sunday that its forces battled between 100 and 150 fighters in two incidents in the province of Uruzgan after coming under attack on Saturday.
ISAF, which is drawn from Nato countries, was fighting alongside Afghan National Army (ANA) soldiers.
The ISAF statement said: "In a firefight lasting several hours, the insurgents were engaged by ISAF and ANA small-arms fire, attack helicopters and close air support.
"Initial battle damage assessment indicates that up to 55 insurgents were killed," it said.
The figure could not be independently verified.
An Afghan soldier was wounded in the battle. ISAF has stationed Australian and Dutch soldiers in Uruzgan province.
In the second encounter, an ISAF convoy was struck by a bomb in the same province. One soldier was killed and wounded according to a statement.
The force does not release the nationalities of its casualties until this information has been made public by the home countries.
ISAF has 31,000 troops from 37 countries in Afghanistan. Its forces have faced solid resistance from Taliban fighters, particularly in the south and along the border with Pakistan.
Meanwhile, Nato's senior commander apologised for civilian deaths caused by fighting between the Taliban and Nato forces earlier in the week.
US General James Jones said: "Sadly, in asymmetric warfare, when you're battling an insurgency, typically the insurgents do not play by the same rules that we would like to play by."
Human Rights Watch, based in New York, had said on Friday Nato's recent operations have killed dozens of civilians.
Sam Zarifi, the group's Asia research director, said: "While Nato forces try to minimise harm to civilians, they obviously are not doing enough.
"Nato's tactics are increasingly endangering the civilians they are supposed to be protecting and turning the local population against them."
The organisation also criticised the Taliban for "using populated areas to launch attacks on Nato and Afghan government forces".
The comments follow battles between Nato-led troops and fighters in the south that Afghan officials say killed 30 to 80 civilians, including women and children. Nato said its initial investigation found 12 civilians killed.