Taliban rebels killed five border policemen when they ambushed their post in southern Afghanistan amid increasing violence in the build-up to a run-off election next month, officials said Tuesday.

Security sources also said nine Taliban fighters, including a commander, were shot dead in their stronghold of Kandahar during a raid by Afghan forces.

Both incidents took place Monday, the same day that 14 American troops and narcotics agents were killed in two helicopter crashes in one of the deadliest days for the United States since they toppled the Taliban in 2001.

The attack on the border post was carried out in the Shamalzai district of southern Zabul province which borders Pakistan.

- Five border policemen were killed and one was wounded - said General Abdul Raziq, the area commander, adding that four Taliban rebels were killed in ensuing clashes.

In Kandahar, which neighbours Zabul, the provincial police chief said Afghan soldiers and police conducted a joint anti-Taliban operation late Monday.

- The forces conducted the operation in Arghandab district that resulted in the killing of nine armed opposition members, including a group commander - General Sardar Mohammad Zazai said.

The Taliban is waging an increasingly virulent insurgency, particularly in their southern heartland, where attacks and threats helped limit turnout in some areas to just five percent in the first round of Afghanistan's presidential election.

Almost 200 violent incidents around the first vote were attributed to the Taliban, including reported amputations of fingers marked with purple ink as proof of voting, and rocket and grenade attacks on polling stations.

The movement has called for a boycott of the run-off on November 7, threatening violence against those who do cast their ballots.