The pro-Assad assault on southern Syria continued on Thursday in advance of a possible ground offensive, displacing thousands of residents.

Bombardment by pro-Assad forces has surged since Monday, particularly in eastern Daraa Province, part of the area near the Jordanian border and Israeli-occupied Golan Heights which the Assad regime has pledged to recapture.

Local sources estimated that more than 12,000 people have fled from the town of Busra al-Harir and nearby areas.

Regime media covered the latest attacks by declaring — falsely — that they are on positions of the jihadist bloc Hay’at Tahrir al-Sham and that rebels fired rockets which killed two people in regime-held Sweida Province to the east. Pro-opposition activists said the casualties were regime military personnel.

The regime has looked towards the reoccupation of Quneitra and Daraa Provinces, where the Syrian uprising began in March 2011, since it reclaimed East Ghouta near Damascus in April.

Israel has warned that it will intervene against any offensive including Hezbollah and Iranian-led foreign militia. After discussions with the Israelis, Russian Foreign Minister Sergey Lavrov said all “non-Syrian forces” must leave the area and suggested a political process for rebel withdrawal. The US and Russia have also declared a “de-escalation zone” across southern Syria since last July.

The Assad regime’s military has not made a significant advance since 2012 without Iranian-backed forces on the ground and Russian airpower. But opposition activists warn that Hezbollah and Iranian-backed militia may disguise themselves within the regime forces, and Russia — while not carrying out any operations so far — has broken other de-escalation zones, notably in East Ghouta and in parts of northwest Syria.

The US State Department, for the third time in recent days, warned of “serious repercussions” of any offensive.

“Syrian regime military and militia units, according to our reports, have violated the southwest de-escalation zone and initiated air strikes, artillery, and rocket attacks,” spokeswoman Heather Nauert said on Thursday.

Jens Laerke, the UN humanitarian spokesman in Geneva, expressed concern “about reports of an escalation of violence in Daraa governorate…endangering civilians and displacing hundreds of families”.