* Assad has vowed to retake southwest from rebels
* United States has warned against gov't moves there
* Southwest is of strategic interest to Israel
Syrian government forces and their Iran-backed allies will face "volcanoes of fire" if they launch a threatened offensive in the opposition-held southwest, a rebel commander told Reuters on Tuesday.
Syria's southwest has come into focus since President Bashar al-Assad and his allies crushed the last remaining rebel pockets near Damascus and Homs.
Assad has vowed to recover opposition-held areas near the frontiers with Jordan and the Israeli-occupied Golan Heights, and government and allied forces are mobilising. A major flareup there risks escalating the seven-year-long war that has killed an estimated half a million people.
Violence flared in several parts of the southwest on Tuesday, with government warplanes launching air strikes near a rebel-held village. But there was no sign yet of the start of the big offensive threatened by the government, the Syrian Observatory for Human Rights war monitor said.
The United States last week warned it would take "firm and appropriate measures" in response to Syrian government violations of a "de-escalation" agreement that it underwrote with Russia last year to contain the conflict in the southwest.
"Everyone is on guard. We are still committed to the de-escalation agreement but if the regime launches any attack on any sector of the south, it will be faced by volcanoes of fire," Nassim Abu Arra, commander of one of the main Free Syrian Army groups in southern Syria, the Youth of Sunna Forces, said.
Rebels attacked a military convoy bringing reinforcements to the area overnight in the Khirbat Ghazala area, igniting clashes between midnight and 2 a.m., he said.
The air strikes near al-Masika village were a response to a separate rebel attack that destroyed a tank, he added.
Syria's state-run Ikhbariya television said a child was killed in an insurgent missile attack in the same area. State news agency SANA said rebel shells had also fallen on Deraa city, killing one girl overnight, and on Sweida city, causing material damage.
Families fled the rebel-held town of Busra al-Harir, fearing it could be targeted, activists said.
The conflict in the southwest has been complicated by the role of Iran-backed forces and Israeli demands for them to kept away from the occupied Golan Heights and, more widely, to be removed from Syria entirely.
Assad said earlier this month the government, at Russia's suggestion, was seeking to strike a deal in the southwest similar to agreements that have restored his control of other areas through withdrawals of rebel forces.
But he also said there had been no results yet and blamed "Israeli and American interference". He said the territory would be recovered by force if necessary.
Abu Arra said the reinforcements arriving in the southwest aimed to put pressure on rebels to succumb to government demands such as accepting "reconcilation" deals, or to surrender strategic positions including the Nassib crossing with Jordan.
"But we have made up our minds. There will be no retreat from the principles of the revolution or surrender of a single inch of the Syrian south," he said.