ISLAMABAD, Nov. 18 (Xinhua) -- A security official was killed and four others were injured when Pakistan's security forces continued their offensive against militants in northwestern Pakistan's Swat valley on Tuesday, local television reported.

   &;nbsp;The local militants ambushed a convoy of the security forces in Kabal area of Swat valley in North West Frontier Province (NWFP), killing one security official and injuring four others, private Geo TV channel said.

    Pakistan's security forces have intensified military operation in the mountainous Swat valley since late July.

    Swat valley was once a tourist region dubbed the "Switzerland of Pakistan" but has been a stronghold of pro-Taliban militants for one and a half years.

    In response to the military operation, the militants retaliated with a wave of suicide attacks.

    On Monday, a suicide bomber rammed an explosives-laden car into a check post in the Swat valley, leaving four security personnel dead and three others injured.