Dark clouds of smoke rose over Syrian rebel-held areas near the border with Jordan on Thursday as President Bashar al-Assad's Russian allies unleashed heavy air strikes and government forces launched ground assaults.
Assad aims to recapture the entire southwest including the frontiers with the Israeli Golan Heights and Jordan. The area is one of the last rebel strongholds in Syria after more than seven years of war.
With no sign of intervention by his foreign foes, Assad seems set for another big victory in the war after crushing the last remaining rebel bastions near Damascus and Homs.
State television footage showed giant clouds of smoke towering over fields, rooftops and a distant industrial area, accompanied by the sound of occasional explosions.
After four days of reduced bombardment, intense air strikes resumed on Wednesday following the collapse of talks between insurgent groups and Russian officers that were brokered by Jordan.
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"The Russians have not stopped the bombardment," Bashar al-Zoubi, a prominent rebel leader in southern Syria, told Reuters in a text message from the Deraa area, the focus of the government offensive so far. "The regime is trying to advance and the clashes are continuing."
The Syrian Observatory for Human Rights, monitoring the war through many sources on the ground, said there had been 600 air strikes in 15 hours extending into Thursday's early hours.
The two-week offensive, backed by massive Russian air power, has taken a large chunk of rebel territory northeast of the provincial capital of Deraa, as a string of towns surrendered.
The fighting and air strikes have already driven around 320,000 people from their homes, including 60,000 in camps along the border with Jordan, the U.N. refugee agency UNHCR said on Thursday. The Observatory said 150 civilians have been killed.
State TV said Thursday's bombardment had targeted the southern parts of Deraa, a city long split between rebels and the army, and the towns of Saida, al-Nuaima, Um al-Mayadan and Taiba.
Its correspondent said the army aimed to drive southwards through the area immediately east of Deraa city, where rebel territory narrows to a thin corridor along the Jordanian border.
This would split the territory in two.
It would also return the main Nassib border crossing with Jordan to government hands, paving the way for the eventual reopening of a major trade route vital to Assad's hopes of reviving Syria's war-raddled economy.
The army has also been trying for days to reach the Jordanian border in the area immediately west of Deraa, but had not succeeded in attempts to storm an insurgent-held air base there, a rebel operations room Twitter account said on Thursday.
Those who have fled mostly sought shelter along the frontiers with Jordan and Israel. Both countries have said they will not open their borders to refugees - Jordan is already hosting some 650,000 Syrian refugees - but say they have distributed some supplies inside Syria.
Southwest Syria is a "de-escalation zone" agreed last year by Russia, Jordan and the United States to reduce violence.
Near the start of the government's offensive, Washington indicated it would respond to violations of that deal, but it has not done so yet and rebels said it told them to expect no American military help.
Israel sent reinforcements to the Golan frontier on Sunday. This year Israel has stepped up attacks against what it has described as Iranian-backed militias in Syria that are crucial to Assad's war effort.
For the anti-Assad rebels, losing the southwest will shrivel their territory to a region of the northwest bordering Turkey and a patch of desert in the east where U.S. forces are stationed near the border with Iraq and Jordan.