The militants used rocket propelled grenades and homemade mortars to bombard the base, close to Pakistan's border, from several sides.
The attack caused one of the single worst losses of life for foreign troops since operations began in 2001.
It came as international and Afghan forces fought militants in many areas.
One soldier from the US-led coalition was also killed by a bomb in the southern province of Helmand.
On Sunday, US forces said 40 insurgents had been killed in Helmand province in 24 hours. There was no independent confirmation of the report.
Also on Sunday, at least six Afghan security guards escorting a Nato supply convoy were killed in a roadside bombing in Helmand's Gereshk district, according to police.
Insurgents 'hiding'
The nine US soldiers were killed in several hours of fierce fighting at the outpost at Wanat, a mountain village in Kunar province, according to a statement from Nato's International Security Assistance Force (Isaf).
Kunar has seen some of the fiercest fighting of the conflict |
Insurgents pounded the Afghan army and Nato base - which had reportedly only been fully constructed days earlier - with rockets, mortars and machine guns, before breaching the compound and attacking the soldiers from inside.
Isaf and Afghan National Army forces responded with small arms, machine guns, mortars and artillery, before deploying fighter jets and apache helicopters, the statement said.
A further 15 Isaf soldiers were wounded along with four Afghan troops, the statement added.
"It was a complex attack, well organised and planned," said Isaf spokesman Captain Michael Finney.
"It was clear they wanted to overrun the combat outpost. They chose their positions well. It wasn't just an attempt to rush the gate."
There may also have been civilian casualties in the rugged area on the border with Kunar and Nuristan province, according to Afghan officials.
Nato says the rebels suffered heavy casualties. It did not name the attackers but there has been a sharp increase in Taleban attacks in the country, and in that region in particular, although other rebel groups are also known to operate there.
The BBC's Martin Patience in Kabul says Afghanistan's north-eastern border with Pakistan is a well-known trouble spot.
'Wedding procession' attacked
The fighting is close to where US forces were accused of killing 47 civilians in an air strike in Nangarhar province a week ago.
The US military said they were militants.
In a separate incident on Sunday, a suicide bomber killed at least 21 people, many of them children, in a market in the Deh Rawud district of Uruzgan province in the south.
No group has claimed responsibility for the suicide bombing.
According to the US military, the number of Taleban attacks in the east of the country has increased by 40% since last year.
More than 130 foreign soldiers have been killed in Afghanistan this year and recent weeks have seen a sharp rise in insurgency-related violence.
Hundreds more, including many militants and civilians, have also died in 2008.
Isaf currently has 53,000 troops from 40 countries. A separate, smaller US-led military force operates alongside it.Original Source : http://news.bbc.co.uk/1/hi/world/south_asia/7504952.stm