Three Taliban militants were killed on Friday when a roadside bomb they were planting exploded prematurely, as Afghan and foreign troops launched an operation to retake a neighboring Afghan town controlled by Taliban militants, officials said.

The blast occurred in Kandahar, south of Kandahar city on a road frequently used by NATO and police forces, he said.

In Musa Qala, ground forces surrounded the area and airstrikes targeted militants inside the town in the southern province of Helmand, defense ministry spokesman Gen. Mohammad Zahir Azimi said.

"The operation has started," said Maj. Charles Anthony, a spokesman for NATO's International Security Assistance Force.

Taliban militants overran Musa Qala in February, four months after British troops left the town following a contentious peace agreement that gave security responsibilities to Afghan elders. Musa Qala has been in control of Taliban fighters ever since.

Situated in north of Helmand, Musa Qala and the region around it have seen the heaviest fighting in Afghanistan this year. It also is in the middle of the country's poppy-growing belt.

A large Afghan and British force have been massing around the town for weeks, facing an entrenched Taliban force.

For months, NATO and U.S.-led coalition troops have been attacking militants in areas surrounding Musa Qala.

For the Taliban militants the town has became a symbol of their ability to hold ground. Elsewhere, they employ hit-and-run attacks, or use roadside and suicide bombs to try to weaken President Hamid Karzai's authority.