Several Western embassies in Islamabad received letters on Wednesday containing suspicious powder and threats to poison supplies for NATO soldiers in Afghanistan, officials said.
Islamabad police chief Bani Amin told AFP the embassies had received small packets containing black powder, which had been sent for laboratory analysis.
The letters said the powder was a sample of "poison" that would be hidden in NATO supplies if Pakistan lifts a nearly six-month blockade on convoys carrying supplies for troops fighting the Taliban in neighbouring Afghanistan.
Senior Pakistani security officials told AFP that the French embassy and the Australian and British High Commissions had received suspicious packages for certain, and other diplomatic missions may also have been targeted.
"Embassies have received one sachet each. The problem is that it is in a meagre quantity and difficult even to test. It seems somebody has committed some mischief. We are sending it to a laboratory," Amin told AFP.
He said the substance looked like kohl, a powdered black cosmetic commonly used in South Asia.
A diplomat at one of the embassies said the accompanying handwritten letter was in broken English and threatened to avenge militants killed in Afghanistan by poisoning food supplies in the convoys.
"We received a letter containing greyish powder in a sealed plastic sachet, which we didn't open," the diplomat told AFP, speaking on condition of anonymity.
There was no risk of anyone being contaminated as the powder did not get out of the sealed bag, the diplomat said, adding that it had been sent to police for analysis.
A spokesman for the British High Commission said: "There was an incident at the High Commission which has now be resolved. Nobody was harmed."
Pakistan closed its borders to NATO convoys supplying the war effort in Afghanistan in November after a US air strike inadvertently killed 24 Pakistani soldiers at a border post.
But talks with the US have been under way for weeks to lift the blockade and on Wednesday Pakistan said it had ordered officials to finalise an agreement as quickly as possible.
A date for the reopening has not been announced but Islamabad has signalled President Asif Ali Zardari will attend key talks on Afghanistan in Chicago on May 20-21 after a last-minute invitation from NATO.
Reopening the supply lines is likely to trigger an angry backlash from opposition, right-wing and religious parties keen to exploit rampant anti-American sentiment in an election year.
In February it emerged that an envelope containing anthrax had been sent to the office of Prime Minister Yousuf Raza Gilani in October.