Thousands of Syrians fled rebel-held areas in the south on Thursday after heavy shelling by regime forces, undermining an international "de-escalation" agreement backed by the US ahead of a possible offensive.
The UK based Syrian Observatory for Human Rights said over 12,000 civilians left the town of Busr al-Harir and nearby areas of Deraa province in the last two days. Rebel officials gave conflicting assessments of the scale of displacement.
"Civilians fleeing were heading to nearby villages under rebel control not affected by the bombardment near the Jordanian border" to the south, the head of the observatory, Rami Abdel Rahman said.
Syrian President Bashar Al Assad has vowed to retake rebel-held areas in the south, whether through negotiations or military means, and has been assembling forces to launch a push.
Washington, Moscow and Amman negotiated a truce for the area last year, which borders Jordan and the Jordan and the Israeli-occupied Golan Heights. The US has said it will take measures to protect the agreement. The agreement includes Deraa, Quneitra and Sweida.
Opposition fighters control around two-thirds of Deraa, which borders Jordan, but the regime holds a sliver of territory in the centre of the province.
ISIS also holds a pocket of ground in the province.
The areas in eastern Deraa bombarded in recent days lie on a strip of land flanked by regime-held territory to the east and west.
OCHA said the United Nations was "concerned about reports of an escalation of violence in Deraa … which is endangering civilians and causing hundreds of families to become displaced".
Deraa is largely considered to be the birthplace of the Syrian uprising in 2011 that eventually spiralled into civil war after a brutal crackdown, and the regime retaking it would be a symbolic victory.
The De-escalation zones include parts of Deraa and the neighbouring province of Quneitra to the west, which is largely controlled by rebels.
Mr Al Assad told Iranian TV that contacts were ongoing between Russia, the United States and Israel over the southern front.
"We are giving the political talks a chance, but if they fail, there will be no choice but liberation by force," he said.
Meanwhile, Turkish Foreign Minister Mevlut Cavusoglu, said on Thursday that Syrian Kurdish YPG fighters will begin leaving northern Syria's Manbij region as of July 4.
Mr Cavusoglu told CNN Turk that the roadmap for a solution in Manbij that was agreed upon by the United States and Turkey is being implemented fully.
Ankara and Washington endorsed a tentative deal, in which the Kurdish YPG militia would withdraw from Manbij and Turkish and US forces would jointly maintain security and stability there.
The city became a flashpoint after Turkey carried out military operations to create a YPG-free zone in northern Syria. The Turkish government considers the group to be a terrorist organisation.
The talk of the pullout comes just days after the Turkish president Recep Tayyip Erdogan announced the first joint patrol with US forces around the town.
However, after the patrol, a spokesman for the Syrian Democratic Movement — the political body that effectively controls Manbij and most of the areas where the US military operates — said that the Turkish military would not enter the city.
“We have been informed of the outline of the agreement, but the details are not clear. What we know is that the city will be managed by its people,” Mr Ahmad said.
Syria's war has killed more than 350,000 people and displaced millions since it started in 2011 with the brutal repression of anti-regime protests.