The several hours of fighting also left 15 US soldiers and four Afghan colleagues wounded, Nato said.
It added that the attackers – whom it did not identify - were repulsed from the outpost and were believed to have suffered heavy casualties.
Captain Mike Finney, a spokesperson for the ISAF, told Al Jazeera that while it was a very bad day for the force in terms of numbers killed, the "insurgents haven't gained any ground".
He said the attackers had failed in their goal to overrun the outpost which the ISAF soldiers had only recently occupied.
An Afghan official said international aircraft had bombed the area during the fighting and there may also have been civilian casualties.
The US-led coalition which works alongside ISAF and the Afghan forces, announced meanwhile it had killed at least 40 fighters in an operation that started on Saturday and was still under way in the southern province of Helmand.
An ISAF soldier was killed in the same area by a bomb on Sunday.
Suicide attack
Elsewhere in the country meanwhile, at least 24 people, including four policemen, were killed in a suicide attack in a bazaar in Afghanistan's southern province of Uruzgan.
The bomber targeted a police vehicle in Deh Rawud district, about 400km southwest of the capital Kabul, Juma Gul Hemat, the provincial police chief said.
"We now have 20 civilians and four police officers killed ... In the hospital we have 27 people wounded," Hemat said.
There was no immediate claim of responsibility for the bombing. Similar attacks have been carried out by the Taliban.
One shopkeeper in the bazaar, who gave his name only as Fazlullah, said it was very crowded when the bombing took place.
"Most of the casualties are shopkeepers and people and children who were selling stuff on the roadside," he said.
Provincial health chief Khan Agha Miakhail said that of those taken to hospital, four were children under the age of 10.
The blast took place less than a week after a suicide bombing at the Indian embassy in Kabul, which killed more than 40 people including four Indian nationals, two of them senior diplomats.
The Kabul bombing was the deadliest suicide attack in the capital since the Taliban were removed from power in 2001.
Women murdered
Also on Sunday, the Taliban said it shot dead two women who it alleged were prostitutes and had worked for the police.
The government, however, denied that the women were working for the police.
Ismail Jahangir, a provincial government spokesman, said the bodies of the women were found late on Saturday in the central province of Ghazni.
In Nuristan province, four police were missing after clashes with fighters near the eastern border with Pakistan, a governor said on Sunday.
Fighters overran two police posts in days of heavy fighting near the border, said Abdul Aleem, the deputy provincial governor.
Two of the fighters, whom Afghan officials said had attacked bases in Pakistan, were also killed, Aleem said.
He identified them as Chechens but this could not be confirmed.
Separately, Al Jazeera's James Bays, reporting from Kabul, said an Afghan MP had been kidnapped by Taliban fighters in Logar province, 80km south of Kabul.
He said the kidnapping had been confirmed by parliamentary sources but the Taliban had not released any conditions for the MP's release yet.