At least 26 pro-regime fighters, most of them Iranian, were killed in overnight missile strikes in central Syria, a monitor said on Monday, as the Syrian army began an intense bombardment of a rebel enclave near Homs.
The UK-based Syrian Observatory for Human Rights said that a military base of the government’s 47th brigade was targeted late on Sunday, in an assault that bore the hallmarks of an Israeli operation.
"At least 26 fighters were killed, including four Syrians," said Observatory chief Rami Abdel Rahman. “The others are foreign fighters, a vast majority of them Iranians.
"Given the nature of the target, it is likely to have been an Israeli strike.”
Mr Abdel Rahman also said that the strikes hit an air base in nearby Aleppo province where surface-to-surface missiles were stored.
On Sunday, Syrian state media denounced a "new aggression" following reported raids by "enemy missiles".
The latest strikes come amid heightened tensions in Syria after Damascus and its ally Iran accused Israel on April 9 of conducting deadly strikes against a military base in the centre of the country.
Meanwhile, President Bashar Al Assad has been pushing to recapture all the remaining areas his forces have besieged.
Syrian state television broadcast footage on Monday of work to clear roads into part of a pocket in south Damascus where fighting had raged, in order to allow some rebels there to withdraw in a surrender deal, it said.
The Syrian army's assault on the pocket between Homs and Hama - the most populous remaining besieged area in Syria - included air strikes and artillery, said the Observatory.
It added that reinforcements arrived in government-held areas before the bombardment, which targeted Rastan, the biggest town in the pocket, and several nearby villages.
Syrian rebels hold large swathes of both northwest and southwest Syria. An alliance of Kurdish and Arab militias backed by the United States holds large parts of northern and eastern Syria after an offensive against ISIS last year.
On Sunday, that alliance, the Syrian Democratic Forces (SDF), said it had recaptured several villages that pro-government forces had seized from it in eastern Syria.
Rebel territory in southwest Syria, near the borders with Jordan and Israel, is part of a so-called "de-escalation zone" agreed by the US and Jordan with Russia.
On Monday, US Secretary of State Mike Pompeo gave assurances of Washington's continued support for the zone, saying in Amman its strategy "remains unchanged".
Jordan's Foreign Minister Ayman Safadi said the zone, covered by a ceasefire, had saved lives and helped to mitigate risk. The area's proximity to Israel has caused concerns it could lead to a wider escalation involving Israel and Iran.
The pocket in southern Damascus is split between areas held by ISIS and rebel groups. It has been the focus of a massive bombardment and intense fighting since the Syrian army recaptured Eastern Ghouta earlier this month.
Late on Sunday, state media reported that one of the rebel groups there, Tahrir Al Sham alliance, which includes Syria's former Al Qaeda affiliate, had agreed to withdraw to opposition-held Idlib province in northern Syria.
Footage on state television on Monday showed what it said was preparations for that withdrawal, along with the departure of buses in northern Syria to evacuate civilians from two government-held villages besieged by insurgents.
The surrender deal for Tahrir Al Sham in south Damascus was part of a deal to allow about 5,000 people to leave the two government-held Shiite villages, Al Foua and Kefraya, it said.
However, SANA state news agency reported continued intensive bombardment of Al Hajar Al Aswad, another area in the south Damascus pocket where ISIS fighters are based.