State media said the army took over the northern city of Tabqa and its adjacent dam, as well as the major Freedom dam, formerly known as the Baath, west of the Syrian city of Raqaa.
Syrian Kurdish authorities had not acknowledged their loss of control over those strategic points, and it was unclear if fighting was still ongoing.
For days, Syrian troops had amassed around a cluster of villages that lie just west of the winding Euphrates and had called on the Kurdish-led Syrian Democratic Forces (SDF) stationed there to redeploy their forces on the opposite bank of the river.
They have been clashing over strategic posts and oilfields along the Euphrates River.
SDF fighters withdrew from the area early on Saturday as a gesture of goodwill - but then accused Syrian troops of violating the agreement by continuing to push further east into towns and oilfields not included in the deal.
Brad Cooper, who heads the US military's Central Command, said in a written statement posted on social media platform X that Syrian troops should "cease any offensive actions in areas" between the city of Aleppo and the town of Tabqa, approximately 160 kilometres further east.
The initial withdrawal deal included the main town of Deir Hafer and some surrounding villages whose residents are predominantly Arab.
The SDF withdrew on Saturday and Syrian troops moved in relatively smoothly, with residents celebrating their arrival.
"It happened with the least amount of losses. There's been enough blood in this country, Syria. We have sacrificed and lost enough - people are tired of it," Hussein al-Khalaf, a resident of Deir Hafer, told Reuters.
The Syrian Petroleum Company said that the nearby oilfields of Rasafa and Sufyan had been captured by Syrian troops and could now be brought back online.
SDF forces had withdrawn east, some on foot, towards the flashpoint town of Tabqa - downstream but still on the western side of the river and near a hydroelectric dam, a crucial source of power.
But when Syria's army announced it aimed to capture Tabqa next, the SDF said that was not part of the original deal and that it would fight to keep the town, as well as another oilfield in its vicinity.
Syria's army said four of its troops had been killed in attacks by Kurdish militants, and the SDF said some of its own fighters had been killed, but did not give a number.
US-led coalition planes flew over the flashpoint towns, releasing warning flares, according to a Syrian security source.
The US has had to recalibrate its Syria policy to balance years of backing for the SDF - which fought against the Islamic State - against Washington's new support for Syrian President Ahmed al-Sharaa, whose rebel forces ousted dictator Bashar al-Assad in late 2024.
To try to end the fighting, US envoy Tom Barrack traveled to Erbil in northern Iraq on Saturday to meet with both Abdi and Iraqi Kurdish leader Masoud Barzani, according to two Kurdish sources. There was no immediate comment from Barrack's spokesperson.
The latest violence has deepened the faultline between the government led by Sharaa, who has vowed to reunify the fractured country after 14 years of war, and local Kurdish authorities wary of his Islamist-led administration.
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