The violence has already ravaged civilian infrastructure in opposition towns, with three hospitals forced to shut down because of heavy raids.
As the humanitarian situation grows increasingly dire, aid organizations from the Red Cross to UNICEF called for more to be done to prevent civilian casualties.
The south is meant to be protected by a ceasefire put in place last year by Russia, Jordan, and the United States, but President Bashar al-Assad has set his sights on retaking the area.
After a week of air strikes and artillery fire on rebel towns across Daraa province, his troops turned to the opposition-held half of the provincial capital on Tuesday.
The bombing of rebel positions in the south continued into Wednesday, the Syrian Observatory for Human Rights said.