The Pentagon will not make the remaining military reimbursements to Pakistan for fiscal year 2016 after US Defence Secretary Jim Mattis told Congress that Islamabad had not taken sufficient action against the Haqqani network, a US official said on Friday.

"The funds could not be released to the Government of Pakistan at this time because the secretary could not certify that Pakistan has taken sufficient action against the Haqqani Network per the requirement in the FY 2016 National Defence Authorisation Act," Adam Stump, a Pentagon spokesman, told Reuters.

The decision comes as US President Donald Trump's administration is exploring potentially hardening its approach toward Pakistan to crack down on militants launching strikes in neighbouring Afghanistan.

"This decision does not prejudge the conclusions of the White House review of South Asia strategy, which is still ongoing," Stump said.

Relations between the two countries have been frayed over the past decade, with US officials frustrated by what they term Islamabad's unwillingness to act against Islamist groups such as the Afghan Taliban and the Haqqani network.

This is not the first time the Pentagon has decided not to make military reimbursements. Last year, the Pentagon withheld $300 million in reimbursements.

Pakistan has been reimbursed $550 million of the $900 million the country was authorised to receive in fiscal year 2016.

Out of remainder, $300 million had already been reprogrammed for other purposes, but had not been previously reported. Mattis' latest decision affects the remaining $50 million.

The amount is under the Coalition Support Fund (CSF), a US Defence Department programme to reimburse allies that have incurred costs in supporting counter-terrorist and counter-insurgency operations.