UPDATE 4-Pakistan orders offensive against Taliban chief

(For coverage of Pakistan and Afghanistan click on [nSP102615])

* Government orders offensive against Mehsud

* Bomb in market kills eight

* U.S. drone strike kills three

* Air strikes, artillery shelling stepped up against rebels (Recasts with governor's comment on offensive)

By Kamran Haider

ISLAMABAD, June 14 (Reuters) - Pakistan has ordered the military to carry out an offensive against Pakistani Taliban leader Baitullah Mehsud and his fighters, a provincial governor said on Sunday.

The announcement came hours after a bomb in a market killed eight people in a northwest Pakistani town, the latest in a wave of attacks since the army launched an offensive against Taliban militants in the Swat valley northwest of Islamabad.

Nuclear-armed Pakistan is struggling to push back a growing Taliban insurgency and security forces have made progress in more than a month of fighting against militants in Swat.