Six SDF fighters were killed, according to the Syrian Observatory for Human Rights, a UK-based conflict monitor.
Syrian state news agency SANA says forces loyal to President Bashar al-Assad, which control territory on the west bank of the Euphrates River, seized four villages from the SDF on the east bank.
SANA claims the Syrian Arab Army “liberated four villages east of the Euphrates River from the so-called Syrian Democratic Forces on Sunday.”
The villages are Al-Jnenah, Al-Jee’a, Shamra al-Hisan and Hawiqa al-Maishiya, according to the news agency.
The SDF claimed on Sunday night it had launched a counter-attack and retaken all four villages.
Both sides are involved in anti-ISIS operations in the oil-rich province. This is the first time the Syrian regime has launched an operation to take SDF territory, the observatory reports.
Earlier on Sunday, SDF spokesman Mustefa Bali reported an “exchange of gunfire.”
“The Syrian regime army along with its militias started to attack our forces in Deir ez-Zor countryside along the Euphrates River,” the alliance said in a statement, AFP reports.
Kino Gabriel, an SDF spokesman, said: “The Syrian regime army, along with its militias, started to attack our forces in the Deir ez-Zor countryside along the Euphrates River in order to block the launch of our operation against terrorism.
“Since yesterday evening until now, the Syrian regime forces, militias and allies are targeting our forces’ positions along the contact lines and trying to advance towards the already liberated villages. Our forces continue to respond to the source of fire in self-defense.
“We, as the Syria Democratic Forces (SDF), reaffirm our commitment and determination to pull up terrorism by its roots. We also affirm our right of self-defense. We consider this aggression and attack by the Syrian regime to be in support of terrorism and an attempt to block the efforts made to defeat terrorism.”
The SDF, backed by the US-led coalition, has liberated much of northern Syria from ISIS control.
Sergei Lavrov, the Russian foreign minister, told a press conference on Saturday that Syria must not be divided and its territorial integrity should be maintained.
Russia is the Syrian regime’s main international military sponsor.
