WASHINGTON (Reuters) - The United States will withhold $50 million remaining in military reimbursements to Pakistan for fiscal year 2016 because Defense Secretary Jim Mattis believes that Islamabad has not done enough to blunt the Islamist militant Haqqani network, a U.S. official said on Friday.

U.S. Defense Secretary James Mattis gestures to the media at the Pentagon in Washington, U.S., April 11, 2017. REUTERS/Yuri Gripas

The decision was not the first time that the Pentagon has declined to make military reimbursements to Pakistan. Last year, it withheld $300 million. Pakistan has been reimbursed $550 million of the $900 million it was authorized to receive in fiscal year 2016.

Of the rest, $300 million had already been reprogrammed for other purposes, but had not been previously reported. Mattis’ latest decision affects the remaining $50 million.

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

Adam Stump, a Pentagon spokesman, said the $50 million could not be released now because Mattis “could not certify that Pakistan has taken sufficient action against the Haqqani Network per the requirement in the FY 2016 National Defense Authorization Act.”

President Donald Trump’s administration is exploring potentially hardening its approach toward Pakistan to crack down on militants launching strikes in neighboring Afghanistan.

Possible Trump administration responses being discussed include expanding U.S. drone strikes and perhaps eventually downgrading Pakistan’s status as a major non-NATO ally.

Mattis told reporters on Friday that the decision was not linked to the administration’s broader South Asia strategy, which is still being reviewed.

“This is simply an assessment of the current state of play,” Mattis said. “It is not a policy, it is a reality.”

The United States in 2012 designated the Pakistan-based Haqqani network as a terrorist organization. The year before, U.S. Navy Admiral Mike Mullen, then the top U.S. military officer, caused a stir when he told Congress that the Haqqani network was a “veritable arm” of Pakistan’s spy agency, the Inter-Services Intelligence directorate.

Pakistan argues that it has done a great deal to help the United States in tracking down terrorists and points out that it has suffered hundreds of deaths in Islamist militant attacks in response to its crackdowns.

Pakistan’s ambassador to the United States, Aizaz Ahmad Chaudhry, said that Islamabad had continued to fight militants.

“It may be noted that the funds in question are a reimbursement of the expenses incurred by Pakistan towards achieving our common objectives in the fight against terrorism, and not an assistance,” Chaudhry said in a statement.

Pentagon spokesman Stump said the decision did not “reduce the significance of the sacrifices that the Pakistani military has undertaken over previous years.”

He said Pakistan still had time to take action against the Haqqani network in order to receive reimbursements next year.