Four alleged smugglers, who were using camels to transport the drugs, have been arrested.

"It appears to be the first large quantity of heroin seized with origins in Afghanistan," one officer told the Associated Press.

Another said it could be the biggest seizure of pure heroin in Pakistan's history.

Correspondents say the drugs haul is a strong indication that large-scale heroin production has resumed in Afghanistan.

The country was the world's biggest producer of the drug until the Taleban authorities destroyed all poppy fields in areas under their control two years ago.

The fundamentalist regime collapsed in December after intense US bombing.

Afghan origins

Poppy production flourishes in the "Golden Crescent", which takes in Afghanistan, Iran and Pakistan.

Pakistan has been declared poppy-free

Pakistan was a major heroin-producing country until 1998 when, with US backing, the anti-narcotics force clamped down on poppy farmers in the country's north-western tribal areas.

The United Nations declared the country poppy-free in 2000.

Pakistan has an estimated two million heroin addicts, more than any other country in the world.

The United Nations International Drug Control Programme (UNDCP) estimates that Afghanistan produced 3,276 tonnes of opium, out of a worldwide total of 4,691 tonnes in 2000.

Heroin is synthesised from morphine, one of the main constituents of the sap of papaver somniferum - the opium poppy.