There was no immediate claim of responsibility, but the Taliban have vowed to step up attacks on Pakistani security forces in the region to avenge a fresh offensive against Islamist sanctuaries in the tribal district of Mohmand.

"A roadside improvised explosive device exploded as an army vehicle passed by in Janikhel town, killing two soldiers and wounding four others," a senior military official told AFP.

An intelligence official confirmed the incident and casualties, about 150 kilometres (95 miles) south of Peshawar, the capital of the northwest.

Just north of Peshawar, another roadside blast struck a police van, killing one policeman in the Mithra area.

Police official Muhammad Ejaz said three other policemen were injured.

On the outskirts of Peshawar, a bomb planted in a NATO oil tanker exploded in a massive ball of fire, destroying 16 vehicles including three other tankers used to supply NATO forces, police said.

"The bomb was planted in a NATO oil tanker. Sixteen vehicles were completely destroyed in the fire caused by the explosion," police official Shafiullah Khan told AFP.

Six people were hurt and six nearby shops damaged in the fire, Khan said.

The tankers and other vehicles were parked at the time of the explosion and had been due to continue the journey into the lawless tribal district of Khyber and across the border into Afghanistan, he added.

NATO oil tankers are regularly attacked in Pakistan by Taliban and Islamist militants fervently opposed to the United States.

Washington says wiping out the militant threat in Pakistan's northwest tribal belt is vital to winning the war in Afghanistan and defeating Al-Qaeda.