The US Defence Department has not withheld $1 billion in funding from Afghan security forces despite Secretary of State Mike Pompeo's vow on March 23 to cut that sum "immediately", five sources familiar with the matter said.

The Pentagon has been reluctant to shave the funds announced by Pompeo and Defense Secretary Mark Esper has not provided guidance to his agency on how to carry it out, three sources told Reuters.

It was unclear why the cut has not been made, whether President Donald Trump's administration may have decided not to reduce the funding - no such policy change has been announced - or if there was some disconnect between the State and Defense Departments.

The White House National Security Council referred queries to the Defense and State Departments. The State Department did not immediately respond to requests for comment. A Pentagon spokesman did not directly address why the cuts had not been made immediately, and said that the State and Defense Departments were working on how to carry them out.