Syrian government and Russian warplanes are reportedly pounding rebel-held areas of south-west Syria, after rebels said talks to reach a truce had failed.
A monitoring group said there had been more than 600 air strikes and barrel bombings in Deraa province overnight.
The attacks intensified after rebels rejected the terms of a ceasefire proposed by the Russian military.
At least 320,000 people have fled their homes since the government launched an offensive in the region two weeks ago.
Many of the displaced are stranded in makeshift camps along the borders with Jordan and the Israeli-occupied Golan Heights. Both countries have refused to allow mass influxes of refugees, sparking fears of a humanitarian crisis.
Almost 150 civilians are also said to have been killed in the fighting.
Deraa and neighbouring province of Quneitra had been relatively calm for almost a year because of a “de-escalation” agreement brokered by the US and Jordan, which support the opposition, and Russia, a staunch ally of the government.
But Mr Assad set his sights on regaining full control of the south-west after defeating rebels in the Eastern Ghouta region outside Damascus in April.
The army and allied militias have made significant gains since launching the offensive in the region on 19 June with the support of Russian aircraft.