One of the last rebel-held areas has come under sustained attack from Syrian air and ground forces. Scores of displaced want to escape into Jordan, which has closed its borders; the UN warns the numbers are rising fast.
More than 600 airstrikes in 15 hours since Wednesday evening have been reported in Daraa province in southern Syria near the border with Jordan.
"The Russians have not stopped the bombardment," Bashar al-Zoubi, a prominent rebel leader in southern Syria, told Reuters. The strikes were stepped up after rebels and Russian officials failed to agree terms for a ceasefire.
The UN refugee agency has called on Jordan to open its borders to people fleeing the fighting, saying 320,000 are displaced and 60,000 have gathered at the border.
United Nations High Commissioner for Refugees Filippo Grandi called on "all parties to redouble efforts to cease hostilities, to allow humanitarian actors to deliver life-saving assistance, shelter and evacuate the wounded."
Close to the border with Jordan
Syrian state media said government forces had captured Saida, some 10 kilometers (6 miles) east of Daraa. This brings them to within 6 kilometers of the rebel-controlled crossing at Nassib on the border with Jordan.
Rami Abdel-Rahman of the UK-based Syrian Observatory for Human Rights said: "If the regime keeps advancing towards the crossing, I fear for the safety of more than 300,000 people displaced and stranded close to the crossing."
A command center for the rebels reported via Twitter they were resisting the attack, despite the air raids, barrel bombs and rocket barrages.
The Syrian forces are aiming to gain control of the border crossings into Jordan and into the Quneitra area near the Golan frontier with Israel.
Civilians have fled their homes and many have gathered on both the Jordanian and Israeli border crossings.
The assault has been launched despite an agreement by Russia, Jordan and the US last year for southwest Syria to become a "de-escalation zone" in order to limit the fighting.
If rebels do lose control of the region, their remaining territory will become an area in the northwest on the border with Turkey and a desert area in the east near the border with both Iraq and Jordan.
Kurdish-led militias, backed by the US, still control areas in the north and east of Syria.
jm/kms (Reuters, AFP)