KARACHI: Pakistan said Friday it had seized one of its largest ever hauls of heroin, impounding 125 kilograms (275 pounds) of the narcotic worth an estimated $13.75 million on the international market.

The seizure is the largest since May last year when the authorities seized 375 kilograms of the drug.

The authorities examined a refrigerated container at Karachi port and found bags of heroin concealed by solution tape and balloons.

“All 5,400 bags were de-stuffed and thoroughly examined and we found 830 pouches each weighing between 145 grams to 155 grams. The total weight of heroin was 125 kilograms,” Babar Ali Shah, a customs official told reporters.

“We have arrested the clearing agent and one of his employees,” Shah said.

Officials said the heroin was smuggled from neighbouring Afghanistan through Pakistan's northwestern province of Khyber Pakhtunkhwa.

“This is the second biggest heroin seizure in the history of Pakistan,”another customs official Qamar Thalho said, putting the value of the stash at $0.55 million on the local market and $13.75 million on the international market.

He said the narcotic hidden in a container was booked for Malaysia.

Pakistan has more than four million drug addicts in a population of nearly 180 million, according to figures compiled by the ANF, which is responsible for investigating and prosecuting drug offences.

Opium poppy is grown on the Pakistan-Afghanistan border, a region infamous for Taliban and al Qaeda-linked strongholds, and branded the most dangerous place in the world for Americans by US President Barack Obama.

Pakistan has a 2,500-kilometre (1,560-mile) porous border with Afghanistan, which supplies 90 per cent of the world's opium used to make heroin.