The US-backed alliance led by Syrian Kurdish fighters captured the main city in eastern Syria and the main border crossing with Iraq on Friday, taking effective control of Syria's vast eastern desert in two rapid moves.
Two security sources based in eastern Syria said that by Friday afternoon the alliance, known as the Syrian Democratic Forces (SDF), had taken full control of the city of Deir Ezzor, the third city to fall out of President Bashar al-Assad's control in a week.
Omar Abu Layla, an activist from the media platform Deir Ezzor 24 with contacts in the city, told Reuters that Syrian government forces and Iran-backed Iraqi fighters had pulled out of Deir Ezzor before the SDF swept in.
Shortly afterwards, the Syrian Democratic Forces swept through the nearby Albu Kamal border crossing with Iraq, two Syrian army sources told Reuters.
Deir Ezzor city has changed hands several times since Syria's conflict broke out in 2011 after peaceful protests against Assad.
It first fell to opposition forces before the ISIS group captured it in 2014. The Syrian army, backed by pro-Tehran Iraqi factions, retook it in 2017 and held it until Friday.
The SDF advance came as Syrian opposition groups led by Hayat Tahrir al-Sham (HTS), an extremist group formerly affiliated with Al-Qaeda, bore down on the central Syrian city of Homs on Friday.
The opposition had already taken the northern city of Aleppo last week and the city of Hama earlier this week, dealing the biggest blows to Assad in years.
SDF head Mazlum Abdi told reporters earlier on Friday at a press conference in Hasakeh city that his forces had "channels of communication with Hayat Tahrir al-Sham (HTS)," particularly in order to protect Kurds living in Aleppo city.
He said the SDF had not clashed with HTS but that the SDF would defend itself if it was attacked, and that it was in contact with both the US and Russia to protect areas under their control.
Abdi, whose force has clashed with Syrian government forces and allied Tehran-backed Iraqi fighters in the past, said he was surprised to see government forces collapse so quickly amid the opposition assault.
In February, he told Reuters that additional air defenses should be deployed in northeast Syria after six of his fighters were killed in a drone attack blamed on pro-Iran factions.
