The fighting started in parts of Deir Az Zor province after the SDF detained a senior commander, Ahmad al-Khbeil, better known as Abu Khawla, who was accused of corruption.

Arresting Abu Khawla angered the rest of the Deir Az Zor Military Council, a militia that had fought as part of the United States-backed SDF since 2016 in its years-long battle against ISIL (ISIS) in Syria.

The SDF controls a semi-autonomous zone in Syria’s northeast, which includes large parts of Deir Az Zor province and stretches into parts of Aleppo in the northwest.

Both the SDF and the tribal militias deny that Abu Khawla’s detention was the reason behind the escalation of violence.

An SDF spokesman accused Tehran and Damascus of sending tribal militias to wreak havoc in northeast Syria, where most of the nearly 900 US troops in the country are stationed.

Tribal leaders said the clashes broke out because they have long been deprived of their oil wealth after the SDF took control of Syria’s biggest oil wells since the departure of ISIL. The tribal leaders complained that their areas are neglected in favour of Kurdish-majority areas.

Joshua Landis, director of the Center of Middle East Studies at the University of Oklahoma, says the situation is likely to escalate and “the costs will only go up”.

Growing distrust

Several Arab activists and members of the Deir Az Zor Military Council told Al Jazeera the divide was because of “discrimination” by the SDF against the Arab population of the region.

“The arrest of Abu Khawla is not the reason behind the uprising. That was merely a spark for Arab tribesmen to act,” said Abu Hassan al-Dairi, an activist from Deir Az Zor, who claimed that Abu Khawla did not support the Arab tribesmen or respect their leaders.

Adham, a leader in the military council who did not want to share his full name for security reasons, said: “The conflict began with the dominance of Kurdish forces over the region during the war against ISIL. We were promised that the tribesmen, represented by the Deir Az Zor Military Council, would eventually regain control, but that never happened.”

“That’s why a guerrilla war against the SDF eventually began.”

The SDF leadership denies it discriminates against the predominantly Arab population under its rule, blaming ISIL remnants for intimidating locals and preventing the area’s development.

Kurdish journalist and activist Massoud Akko says ethnic divides are not playing a part in the violence. Rather, he said, some Arab tribal leaders, supported by the Syrian government and militias loyal to Iran, have been the root of the problem and are the ones fighting the SDF.

“The SDF is leading a military campaign to end this rebellion, and it will succeed in securing the region in coordination with Arab tribes,” he told Al Jazeera.

No end in sight

The battles have been going on since August 28, and the fighting is no longer confined to Deir Az Zor.

“The violence has reached Al-Hassakeh and is moving towards Raqqa, Tal Abyad, the suburbs of Aleppo and the outskirts of Manbij” in the northwest, Abdel Basit Abdel Latif, a member of the opposition Syrian National Coalition, told Al Jazeera.

Al-Dairi said the fighting has pushed many civilians to flee their homes and move towards villages under the control of the Arab tribes.

“Meanwhile, the majority of the tribal sheikhs continue to reject dialogue with the SDF, especially after it targeted Arab villages, homes and residential areas,” Abdel Latif said.

Arab tribes are in control of swathes of land extending to the Iraqi border, according to activists on the ground. But with lightweight weapons to fight the SDF’s armoured vehicles, tanks and snipers, they may not be able to hold on to those areas for much longer.

Ethnic divide or territorial control?

According to Aron Lund, a fellow at the Century International think tank: “The most obvious divide is ethnic, but it’s not very clear-cut.” He explained that new social and political cleavages as well as intense foreign pressures have played a part in the escalation.

“There are all sorts of political and other interference from the United States, Turkey and the Damascus government [of Bashar al-Assad], backed by Russia and Iran. Plus, of course, [ISIL], which is always hanging around zones of tension looking for recruitable malcontents,” Lund said.

The competition between tribes in Syria goes back centuries and has long revolved around land and water.

“Both [Kurds and Arabs] competed for the best grazing land and the land adjacent to rivers,” said Landis, adding that the competition took on a different tone with the fall of the Ottoman Empire and the rise of nationalism.

“All the peoples of the Jazira region hoped for their own nation states – Turks, Armenians, Assyrians, Arabs and Kurds,” he explained, referring to a region that spans parts of northeast Syria, southeast Turkey and northwest Iraq.

In more recent times, the United States used ISIL’s push to ethnically cleanse Kurds to justify their presence in the area.

“The US stepped into the midst of this national struggle in 2014 when it took the side of the Kurds of Kobani against ISIL. This infuriated the Turks as well as many Syrian-Arab opposition parties,” Landis said.

End to US influence?

It remains unclear where the recent escalation will lead, but analysts believe the violence is likely to intensify and may see an end to US influence in the region.

“The escalation has been startlingly quick and sharp, but the extent of fighting is still somewhat limited, Lund said.

While Lund explained that the SDF’s core Kurdish components would normally have the upper hand militarily and support from Arab tribal fighters who remain loyal to the original configuration, foreign interference could tip this balance. “It is not a stable situation,” he said.

For Landis, the tipping point lies in what will happen to the US presence in the region.

“The US will be unable to square the national ambitions of both groups. It may be able to mediate for a while, … but ultimately, it will withdraw from the region,” Landis said.

“All the neighbouring states, save Israel, want the US out. They will work toward this end by exacerbating the ethnic tensions and stoking the desire of Arab tribes to rule over the Euphrates River basin and Deir Az Zor to get more of the oil revenue,” he added. “America is sitting on a powder keg.”

Additional reporting by Ali Haj Suleiman in Idlib, Syria