A Taliban attack that killed nine US soldiers, the biggest single American loss in Afghanistan since 2005, was a well-planned, complex assault aimed at overrunning an outpost near the Pakistan border, a NATO spokesman said.
The Taliban have largely shied away from large-scale attacks on foreign forces since suffering severe casualties in assaults on NATO bases in the south in 2006. Instead the militants have scaled up hit-and-run attacks and suicide and roadside bombs.
"It was a complex attack, well organised and planned ... it was clear they wanted to overrun the combat outpost," said Captain Michael Finney, spokesman for NATO's International Security Assistance Force (ISAF).
The Taliban began the assault on the Afghan army and ISAF outpost in the mountainous and thickly forested Pech Valley district of Kunar province just before dawn on Sunday, warning villagers to leave their homes before the fighting started.
"They chose their positions well. It wasn't just an attempt to rush the gate," Finney said.
The defenders, numbering between 100 to 150, called in airstrikes from attack helicopters and fixed wing warplanes in fighting that went on till mid-afternoon.
There was no exact count of Taliban dead, but ISAF estimated militant losses to be "in the high double figures, close to triple figures", the spokesman said.
Tens of Taliban were killed, an Afghan Defence Ministry spokesman said.
Spike in violence
There has been a marked increase of violence in Afghanistan this year, especially along the eastern border where militants have effectively secured their rear with de-facto ceasefires with Pakistani forces and launched more attacks into Afghan soil.
The surge in violence is also partly due to the higher numbers of ISAF and Afghan forces in the country entering areas, such as the Pech Valley, where they seldom went before.
Afghan officials said on Sunday the Taliban fighters had infiltrated from nearby Pakistan to launch the attack. However, NATO's Finney said it was unclear whether the militants had crossed the border.
But Taliban sanctuaries in Pakistan were "a matter of great concern", he said.
Afghan leaders are growing increasingly impatient with Pakistan, whose new government has adopted a policy of trying to treat with militant leaders instead of battling them.
Afghan officials have hinted at Pakistani agents being involved in a string of attacks in Afghanistan, including a suicide bomb on the Indian embassy in Kabul last week which killed 58 people and an attempt to kill President Hamid Karzai in April.
Karzai last month threatened to send troops into Pakistan if Islamabad does not take action against militant sanctuaries.
Pakistan denies it is aiding militants and says the Kabul government should try harder to negotiate a peaceful end to the conflict, which it says is an internal Afghan issue.