US president phasing out agency’s programme to arm and train groups in order to get closer to Russia, say US officials.
Activists say 28 Syrian government soldiers have been killed in a firefight after a military unit was ambushed near the capital, Damascus.
The ambush on Thursday followed days of intense fighting as opposition-held suburbs of Damascus have come under a government offensive.
The Britain-based Syrian Observatory for Human Rights (SOHR) said the soldiers and allied fighters were ambushed in the village of Rihan, east of Damascus.
Anas al-Dimashqi, an opposition activist based in the area, and the SOHR said the ambush killed at least 28 troops and pro-government fighters.
Syrian government forces and their allies have been trying to storm the eastern suburbs of Damascus, known as Eastern Ghouta, but have struggled to penetrate opposition defences.
More than 330,000 people have been killed in Syria since the ongoing conflict began with anti-government protests in March 2011.
Elsewhere in Syria, fighters in the northwestern region have gained ground from their Turkey-backed rivals after battling for a second successive day, according to opposition sources.
They said clashes persisted in most of Idlib’s major towns on Thursday between Hay’et Tahrir al-Sham, a faction led by the formerly al-Qaeda-affiliated al-Nusra Front, and its rival Ahrar al-Sham.
Even outside Idlib, Tahrir al-Sham and Ahrar al-Sham – who have thousands of fighters spread across Syria – were reinforcing their positions and putting their forces on heightened alert, according to an opposition source.
Infighting has weakened the opposition since the start of the uprising against President Bashar al-Assad in 2011.
Their turf wars have helped Assad and his allies recover significant ground in the north and around Damascus.
Tensions have been building between Idlib province’s two biggest anti-government factions, mainly over ideological differences.
They have also vied for dominance in the only Syrian province that is entirely under opposition control.
Recent clashes on the outskirts of Idlib towns that rival factions had promised to local councils would be spared from conflict have raised the risk of fighting spilling into these heavily populated urban areas, opposition sources said.
Tahrir al-Sham on Thursday rejected a mediation initiative by prominent religious leaders.
It said the only way to end the fighting was to set up a local regional administration for the opposition-held north that would help reconcile factions and deter foreign meddling in the conflict.