Fighters from the Free Syrian Army ride armoured pick up trucks during battles against ISIS militants near the town of Qabasin.

CIA-coordinated military aid for rebels in northwest Syria has been frozen since they came under major extremist attack last month, rebel sources said, raising doubts about foreign support key to their war against President Bashar al-Assad.

Rebel officials said that no official explanation had been given for the move this month following the militant assault, though several said they believed the main objective was to prevent arms and cash falling into extremist militant hands. They said they expected the aid freeze to be temporary.

The halt in assistance, which has included salaries, training, ammunition and in some cases guided anti-tank missiles, is a response to extremist attacks and has nothing to do with US President Donald Trump replacing Barack Obama in January, two US officials familiar with the CIA-led program said.

The freeze reflects the troubles facing Free Syrian Army (FSA) rebels in the almost in the almost six-year-old revolt against Assad, who now appears militarily unassailable in his core western region largely thanks to direct intervention on his side by Russia and Iran.

“The reality is that you have changes in the area, and these changes inevitably have repercussions,” said an official with one of the affected FSA rebel groups. He said no military assistance could “enter at present until matters are organized. There is a new arrangement but nothing has crystallized yet.”

The CIA declined comment on the reported freeze in support. A Qatari official said his government had nothing to say on the matter. Turkish officials said only they could not discuss “operational details.”

Reuters confirmed the freeze with officials from five of the FSA groups that have been recipients of financial and military support from the so-called “MOM operations room.” It was also confirmed by two other senior FSA figures briefed on the matter.

They spoke on condition of anonymity given the covert nature of the CIA-backed program and the sensitivity of the subject.

Several rebels believed the aid halt was temporary, with new arrangements expected, but there was no clarity yet. Confirming the freeze, two senior FSA sources said donor states were aiming to send the aid to one, unified fighting force – a coherence that has eluded rebels throughout Syria’s civil war.

One of the FSA officials said he did not expect the rebels to be abandoned as they represent the best hope for blocking a further expansion of extremist influence in Syria, and to fight back against the growing role of Iran there.