A Kurdish-dominated militia moved to solidify its control over resource-rich parts of eastern Syria on Thursday after suppressing an Arab tribal insurgency with US support.
Fighting in the area has diminished after the Syrian Democratic Forces, on Wednesday entered the Diban stronghold of a tribal leader who had challenged Kurdish control of the east without resistance.
The centre of Syria's oil and wheat production lie in the region.
A source in the US-backed administration of the east said the SDF has started to hunt down "elements" among the insurgents believed responsible for killing dozens of its members over the past two weeks.
"The rest will be spared," he said.
Officials from Washington were dispatched to the area to solve the crisis after an estimated 200 people were killed. US intervention resulted in a solution to the standoff in Diban, under which the Arab fighters, who mostly belonged to the Okeidat tribe, walked away, along with their leader, Sheikh Ibrahim Al Hafl.
The insurgency has undermined the idea that the US has supported an ethnically harmonious eastern zone united against ISIS. In 2019, the SDF declared victory against the group after overrunning the village of Bahouz in Deir Ezzor governorate under US air cover.
The US formed the SDF in 2015 as the main ground component in the fight against ISIS in Syria, playing on tribal rivalries and Arab-Kurdish competition to promote American influence.
Its Kurdish allies, meanwhile, furthered their grip on the war economy in the east and kept open channels with Iran and President Bashar Al Assad, contributing to Arab resentment against their rule.
The east accounts for most of Syria's oil production, which is running at 50,000 to 100,000 barrels per day, compared with 300,000 bpd in 2010, a year before the country's pro-democracy revolt and the subsequent civil war.
The core of the SDF comprises a Syrian Kurdish militia called the People Protection Units, which originates from the Syrian branch of the Kurdistan Workers Party (PKK), a Turkish Leninist group which has fought Ankara for decades.
Syria has been split into Russian, Iranian, Turkish and US zones in the last decade of the civil war, which started in late 2011, after the authorities put down a peaceful protest movement against the Assad government.
The bulk of the US zone in Syria runs along the Euphrates River basin in the east, adjacent to areas ruled by Turkish and Iranian proxies.
In the first Turkish reaction to the bloodshed in the area, Foreign Minister Hakan Fidan said on Wednesday the US must act “to stop suppressing Arab communities in Syria at the hands of YPG/PKK terror group".
“The painting of YPG terror group as legitimate must end," he said, calling the recent fighting " just the beginning".
Hours after Mr Al Hafl's defeat in Diban, makeshift tribal forces in the Turkish controlled zone in eastern Aleppo governorate launched revenge attacks against SDF positions near the rural city of Manbij.
A tribal commander said six of his men were killed on the Manbij front in the last 24 hours, without making any substantial progress against a well-entrenched enemy.
Like most areas of the east, Manbij is an Arab majority, but is controlled by the SDF.
A senior member of the Syrian opposition said the US had warned Turkey against backing any sustained attacks on the SDF in Manbij or other outlaying areas in the SDF zone.
"Turkey has heeded the American warning," he said from Istanbul. "All the Arabs got from Turkey was rhetoric yesterday."
