The Syrian Observatory for Human Rights said bombing raids, including by Russian warplanes, had damaged medical centres in the opposition-controlled towns of Saida, Al-Mseifra and Al-Jiza in Daraa province.

"The Saida hospital was put out of service after midnight, because of unidentified air strikes near the facility," said Observatory head Rami Abdel Rahman.

Air strikes by planes identified as Russian then hit near the medical centre in Al-Mseifra, damaging it and forcing it to close, he said.

"The hospital in Al-Jiza was damaged this morning. There were Russian air strikes close to the hospital, which damaged it and put it out of service," Abdel Rahman added.

He had no immediate information on the fate of medical staff or the patients inside the facilities.

The recent closures bring to five the number of hospitals that have been put out of service by the Syrian government's week-long military push on the south.

Troops began ramping up air strikes, rocket fire and the dropping of barrel bombs on rebel territory in Daraa and neighbouring Quneitra province on June 19.

Russian warplanes then began striking, and troops launched an assault on the rebel-held half of Daraa city on Tuesday.

State news agency SANA said air strikes on Daraa were a prelude to an "advance into the southeastern quarter of the city."

The Observatory said heavy Russian and Syrian raids, rockets and barrel bombs were still hitting rebel-held neighbourhoods on Wednesday morning.

The Britain-based monitor, which relies on a network of sources inside Syria, says it relies on flight patterns, aircraft involved and ammunition used to determine who carries out air strikes.