A senior anti-narcotics force official, Mukhtar Ahmed, said the suspects were postal workers who sent the drug abroad in envelopes and parcels.

"The gang unearthed at the international mail office in Pakistan had smuggled around 2,000 to 3,000 kg (4,400 to 6,600 lb) of heroin," Mr Ahmed said.

He said the alleged smugglers took wrongly addressed parcels and letters sent to Pakistan, put heroin inside them, changed the return addresses and mailed them back out of the country.

Seven post office employees had been arrested in Karachi, he said.

'Caught red-handed'

"The persons were pinpointed and all of them kept under surveillance. Eventually in December they were apprehended red-handed," Mr Ahmed said.

Pakistan is on a smuggling route for the heroin produced in Afghanistan where poppy cultivation has thrived during the last few years.

Pakistani authorities say they confiscated 1.4 tons of opium, 1.1 tons of heroin, 27 tons of hash and 1.5 tons of other drugs in first six months of 1998.