But "there are a number of technical issues that we are still working through," State Department spokeswoman Victoria Nuland said, as talks continued on reopening lines that have been closed for six months.

Pakistan shut its Afghan border crossings to NATO supplies on November 26 after US air strikes killed 24 Pakistani soldiers.

"We have had some progress (in the talks). We've also had some very helpful and positive political statements out of Pakistan, some political moves," Nuland told reporters.

Pakistani Foreign Minister Hina Rabbani Khar said Monday it was time to "move on" and repair relations with the United States and NATO.

Information Minister Qamar Zaman Kaira said Wednesday "there is no deadline" for the talks, but said "all departments have been asked to conclude their negotiations in the quickest possible time."

"While the Pakistani political leadership hasn't yet authorized the reopening of the ground transportation routes, we understand that they did endorse the conclusion of the negotiations," Nuland said.

She added the Pakistanis "are sending a political signal to their own negotiating team that they would like to see this wrapped up. That doesn't change the fact that we still haven't closed all the issues."

Nuland did not elaborate on where stumbling blocks remained.

But she added: "So they see some urgency, we see some urgency, but the negotiators have to finish it."

When asked if the urgency was to conclude a deal before the NATO summit begins at the weekend in Chicago, she replied: "The urgency is to be able to support Afghanistan from Pakistan."

Although Islamabad has stopped short of announcing when the transit lines will reopen, it has signaled President Asif Ali Zardari will attend talks on Afghanistan in Chicago on May 20-21, after a last-minute invitation from NATO.

The cabinet on Wednesday welcomed NATO's invitation to Zardari, and it is thought unlikely he would be willing to risk the wrath of Western leaders if the supply lines have not been restored by the time of the summit.

By going to Chicago, Pakistan hopes to ease its international isolation and boost its leverage over the future of Afghanistan, as Western countries pull out their combat forces by 2014.

But Islamabad has essentially been forced to climb down on demands for an American apology for the air strikes and an end to drone strikes targeting Taliban and Al-Qaeda on its soil.

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.