Syrian troops launched an offensive Saturday aimed at cutting off the Islamic State's stronghold of al-Raqqa from the Turkish border, a monitoring group said.
Troops and militiamen backed by Syrian and Russian warplanes pushed east from the regime outpost of Ithriya, coming within 40 kilometres of the Islamic State-held city of Tabqa and the main Raqqa-Aleppo road, the Syrian Observatory for Human Rights said.
The offensive comes days after the Kurdish-led Democratic Forces of Syria, backed by US airstrikes, launched an attack further north, also aimed at cutting the jihadists' de facto capital of al-Raqqa off from its access routes to Turkey.
Kurdish-led forces are also pushing towards Tabqa, located near Syria's largest dam, at the southern end of Lake Assad on the Euphrates.
The Kurdish-led forces on Saturday gained control of the most direct remaining route between the city of Minbij, the focus of their current offensive, and al-Raqqa, the Observatory reported.
Observatory director Rami Abdel-Rahman said that the Islamic State's last border territory, the so-called Minbij pocket, faced being surrounded by three hostile parties if the government offensive succeeded.
In addition to the regime forces in the south and the Kurdish-led forces advancing from the east, jihadists in the strategic border territory have been fighting mainly Islamist opposition forces on their western flank.
The Turkish military, meanwhile, said it fired artillery at Islamic State positions in northern Syria, killing 14 jihadists according to the state-run Anadolu news agency.
There was no independent confirmation of the toll. Islamic State has been firing rockets at southern Turkey since January, killing about 20 people.
In Aleppo, Syria's largest city before the war, 35 civilians were killed in rebel shelling and government airstrikes, the Observatory said.
Some 24 civilians were killed in rebel shelling of the city's government-held western sector, and another 11 in airstrikes on the rebel-held east.
Recent government advances, backed by Russian airstrikes, have put rebel-held eastern Aleppo at risk of siege. The Observatory said 468 civilians have been killed since the tit-for-tat shellings and air raids intensified in late April.