The Pentagon has withheld $300 million in military assistance to Pakistan, U.S. officials said Wednesday, a potential blow to U.S.-Pakistani ties and a sign of ongoing frustration with Islamabad for not acting against militants fueling violence in Afghanistan.

Adam Stump, a Pentagon spokesman, said that Defense Secretary Ashton B. Carter had decided against making a certification to Congress stating that Pakistan is taking sufficient action against the Haqqani network, a Taliban affiliate blamed for attacks on U.S. and allied personnel in Afghanistan.

Stump said the decision, which means Pakistan will not receive $300 million in military reimbursement funding, was based on the continuing operations of the Afghan Taliban and Haqqani militants on Pakistani soil.

The move ends a year of speculation about an aid program that has been fundamental to Pakistani military operations, and reveals the strains in Washington’s relations with an ally that many officials have accused of double-dealing with militant groups.