At least three soldiers were killed and six others injured when a bomb hit their vehicle in Pakistan's northwest tribal area of North Waziristan on Sunday, local media reported.

Aaj TV said that some unknown militants blew up a roadside planted bomb with a remote controlled device as soon as the security forces' vehicle passed by it in Datta Khel area of North Waziristan Agency, a restive tribal area along Pakistan- Afghanistan border.

The injured troops have been shifted to military hospital of the neighboring Banu area.

Security forces cordoned off the area for investigations.

No group has claimed responsibility for the attack yet.

The attack happened at the time when the Pakistani army is carrying out an armed offensive against the Taliban in the area.

Named as "Operation Zarb-e-Azb", the offensive was launched on June 15 last year with an aim to remove the sanctuaries of local and foreign militants from the North Waziristan Agency.

The security forces made many achievements in the one-year long operation by taking hold of many important areas and killing over 2,700 militants.

The operation has basically broken the backbone of the militants as the terrorist attacks are decreasing in the country by every coming day.

The militants, however, keep on carrying out sporadic attacks on the security forces whenever chance arises to them.

So far the militants have killed over 250 soldiers including officials in retaliatory attacks on the troops who are carrying out the operation.