WASHINGTON — The United States said Monday it is making progress toward encouraging Pakistan to lift curbs it said the authorities there had imposed on US diplomats traveling in the country.

"There was an incident last week, I believe, where diplomats were prevented from traveling between Islamabad and Peshawar," State Department deputy spokesman Mark Toner told reporters.

"We obviously raised our concerns. We feel that we're making progress towards... resolving the issue," Toner said after saying the issue was raised between US and Pakistani officials in both Islamabad and Washington.

He said that diplomats were later able to travel between the Pakistani capital and Peshawar with a certificate, although he insisted the envoys should be able to travel freely in line with the Vienna Convention.

"We're working cooperatively with the government of Pakistan to resolve the issue," Toner said.

Toner also said the Pakistani authorities asked Cameron Munter, the US ambassador to Pakistan, to produce a certificate of permission to travel before boarding a plane to the southern city of Karachi.

He did not have one, but was allowed to travel there anyway, Toner said without specifying whether it amounted to a diplomatic incident.

Toner declined to rule out US authorities imposing travel restrictions on Pakistani diplomats traveling within the United States.

"Speaking hypothetically or theoretically, reciprocity is always a consideration," he said.

Toner did not say how Washington was informed of the restrictions but a diplomatic source said Sunday that a letter sent to the US embassy in Islamabad increased limitations on when and how diplomats can move outside the capital.

Pakistan is seen as a key ally for the United States in its fight against Islamist militancy, but relations have soured since US troops killed Al-Qaeda chief Osama bin Laden in Pakistan in May without warning Islamabad of the raid.