The Syrian Democratic Forces (SDF) militia took control of most of Diban, near the Euphrates River in Deir Ezzor governorate, after a deal with Arab fighters whom the militia had surrounded, the sources said.
They are led by Ibrahim Al Hafl, a major figure in the Okeidat tribe, one of Syria’s largest. The tribal force concentrated in Diban after Mr Hafl challenged Kurdish control over the mostly Arab area.
“A deal has been reached under which the Okeidat save face,” said a Kurdish source in the SDF-dominated administration in east Syria, without giving details.
Asked about Mr Hafl, the source said “he is on the run”.
The insurgency has exposed defects in US-supervised ethnic alliances set up in east Syria in the past eight years, mainly to fight ISIS.
The east comprises the bulk of the US zone of influence in Syria, adjacent to areas controlled by Turkish and Iranian proxies. But the US zone is responsible for most of Syria's oil and wheat production.
Both dropped sharply since the civil war, which started in 2011, after the authorities used force to suppress a peaceful protest movement against President Bashar Al Assad.
Large parts of Syria have since been fragmented into zones run by factions financed by Iran, the US, Russia and Turkey.
In the east, the administration and local military forces, including the SDF, are ultimately controlled by the People's Protection Units, a Kurdish militia more commonly known as the YPG.
The anti-SDF insurgency started after Abu Khalwa, an Arab tribal ally of the SDF in Deir Ezzor, was arrested last month, prompting calls among the tribes to rid the area of Kurdish militia control, and demands that the United States deals directly with the area, and not through the SDF.
In an effort to resolve the crisis, Washington sent to the area Ethan Goldrich, deputy assistant secretary of state for Near Eastern affairs, and Maj Gen Joel Vowell, who heads the anti-ISIS mission.
The two men met Arab tribal leaders and SDF commanders over several days to discuss local grievances and the “need for de-escalation”, a US State Department spokesman said on Tuesday.
Since it was founded in 2015 under YPG control, the SDF has sought unsuccessfully to build an uninterrupted fiefdom along the border with Turkey, stretching from the north-east to the north-west and covering large areas populated by Arab tribes.
The YPG traces its roots to the Democratic Union Party (YPD), a branch of the Marxist Leninist Kurdistan Workers Party (PKK), which has fought Turkey for decades. The PKK is designated as a terrorist organisation in the US.
Like other foreign powers, Washington has played on historic tribal and ethnic rivalries to strengthen its position in Syria. This has vastly benefitted the YPG, which overran much Arab territory over the past decade.
The Kurdish source said US officials have held negotiations with members of the Shummar tribe and branches of the Okeidat seen as not loyal to Mr Hafl.
A senior figure in the Syrian opposition against Mr Assad said Mr Hafl could emerge politically as an enhanced figure “provided the Kurds don’t manage to assassinate him”.
“The Arab grievances are here to stay and this is the American chance to fix the situation,” he said.
He said the longer the situation goes on, the more receptive Arab tribes will be to receiving help from Iran.