Pakistani authorities have barred a US diplomat involved in a fatal traffic accident from leaving the country, forcing an American military aircraft flown in for his departure to leave without him, local media reported on Saturday.

The move came a day after Pakistan said it would restrict the movements of all American diplomats in the country in response to Washington's similar restrictions on Pakistani embassy diplomats.

A spokesman for the US embassy in Islamabad declined to comment on the media reports, and a US State Department spokesperson in Washington would neither confirm nor deny them.

"For the privacy and security of those involved, we cannot disclose the diplomat's current location," the State Department spokesperson said.

Pakistan is a crucial link to supplying American troops fighting the Taliban in neighbouring Afghanistan. But Washington has long believed it actually shelters the Taliban's leaders, and President Donald Trump has cut off military aid in an effort to pressure Pakistan.

The latest blow to relations came on Saturday, when Pakistani authorities banned a US military attache from leaving as planned, Pakistan's The Nation and Express Tribune newspapers reported.

A day earlier, an Islamabad court had ruled his diplomatic immunity may not apply in the April 7 traffic accident in which the US attache's vehicle hit a motorcycle, killing the 22-year-old driver, both papers reported.

As a result, a US Air Force C130 flown in to Pakistan's Nur Khan air base outside Islamabad was forced to leave without him on Friday, Geo TV and the two newspapers reported.