UPDATE 1-U.S. troops died in Taliban attempt to storm base

(Adds quotes, details)

By Jon Hemming

KABUL, July 14 (Reuters) - A Taliban attack that killed nine U.S. soldiers, the biggest single American loss in Afghanistan since 2005, was a well-planned, complex assault which briefly breached the defences of an outpost near the Pakistan border.

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.

"The insurgents went into an adjacent village, drove the villagers out, used their homes and a mosque as a base from which to launch the attack and fire on the outpost," said NATO spokesman Mark Laity on Monday.

"Some of the insurgents also then attacked. I think it looks as if they made a brief breach into the base and were repelled," he said.

Troops from NATO's International Security Assistance Force (ISAF) and the Afghan army only moved into the combat outpost in the mountainous and forested Pech Valley district of Kunar province days before and the defences were not fully constructed.

The Taliban began their attack just before dawn on Sunday.

After driving back the assault, the defenders, numbering between 100 to 150, called in airstrikes from attack helicopters and warplanes. Fierce fighting went on till mid-afternoon.

Scores of Taliban fighters were either killed or wounded.

"There was very heavy fighting and they suffered very heavy casualties," Laity said.

Tens of Taliban were killed, an Afghan Defence Ministry spokesman said.

CROSS-BORDER THREAT

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 establishing footholds in 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, ISAF spokesman Captain Michael 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 make a treaty with militant leaders instead of battling them.

Afghan officials have accused Pakistani agents of involvement 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. (Editing by Valerie Lee)