AMMAN (Reuters) - Syrian jets struck rebel-held towns in the country’s south on Monday, the first aerial attacks on the area since the United States and Russia reached an agreement making it a “de-escalation zone” last year, rebels and residents said.

At least eight raids struck the rebel-held towns of Busr al-Harir, Hrak, al-Gharaiya al-Gharbiya and al-Sowara in rural areas in eastern Deraa province in southern Syria, two rebel officials told Reuters.

A rebel source said some of the targets hit in the raids were close to a front line in northern Deraa close to a major Syrian army garrison near the government-held town of Izra.

Another strike hit a civil defence centre in the Laja area and residential areas in several towns, a resident in Busr al Harir said.

The south is one of three parts of the country where large populations are still under the control of rebels opposed to President Bashar al-Assad, along with a northern area near the Turkish border and the Eastern Ghouta region on the outskirts of Damascus.

Government forces have focused their efforts on Eastern Ghouta since mid-February, launching one of the fiercest campaigns of the war now entering its eighth year.

Worries are growing in Jordan and Western powers that the Syrian army, backed by Moscow and Iranian-backed militias, will press on with a major assault to regain the south if it retakes Eastern Ghouta, two senior Western diplomats said.

One rebel commander said the strikes in the south appeared to be a warning to rebels under the Free Syrian Army (FSA) umbrella, who were planning to wage an offensive in coming days to relieve pressure on their comrades in Eastern Ghouta.

“We were starting an operation, and we had not announced zero hour, and the regime pre-empted us,” said Abu Nabout, a military commander in Liwa Tawheed al-Jnoob, a rebel faction in the FSA rebel alliance.

Another rebel official said FSA factions were already mobilising fighters for a possible wider showdown.

“I can say all the factions in the south are in a state of full readiness and alertness with all their equipment and fighting force,” said Khaled al-Faraj, the commander of a rebel group operating in Quneitra province.

Russia, which backs Syria’s government in the civil war, and the United States, which has backed rebel forces seeking to topple Assad, met secretly in Jordan in June and announced a ceasefire in Syria’s southwest a month later.

The July 7 truce - the first U.S. peacekeeping effort of the war under President Donald Trump - was expanded last November in the southwestern triangle bordering Israel and Jordan.

Although violations have occurred, it has reduced fighting there and was meant to lead to a longer-lasting de-escalation, a step towards a full settlement.

The U.S. State Department said on Monday it was concerned by the violence and called an “urgent meeting” in Jordan to ensure maintenance of the de-escalation zone.

“If (reports of the strikes are) true, this would be a clear violation of the (southwest) ceasefire by the Syrian regime that broadens the conflict,” a State Department official said.

“We urge all parties in the southwest de-escalation zone not to take actions that would jeopardize the ceasefire and make future cooperation more difficult.”

Rebels have long feared Syria’s army would return to attack them once it has consolidated gains in the north and other areas. Insurgents say the de-escalation zones free Syria’s army to make territorial gains elsewhere.

Syrian rebels say they have been monitoring increased activity by pro-government and Iranian-backed militias in parts of the south, which they suspect are preparations for a future offensive.

Washington has been channelling tens of millions of dollars into bolstering civic institutions and extending humanitarian aid to opposition-run local councils in the south as part of a post-truce stabilisation programme, according to officials.

U.S. ally Jordan is worried about a possible breakdown in the truce and a major flareup in violence, which could send tens of thousands of refugees fleeing fighting in Deraa towards the safety of its northern borders, diplomats say.

Jordan’s King Abdullah praised Russian President Vladimir Putin during a visit to Moscow last month for helping to maintain the truce in southern Syria.

Abdullah fears a spillover into the kingdom in the event of a major battle along its border. His country hosts a monitoring centre run with Russian logistical support that tracks ceasefire violations and has stepped up intelligence cooperation.

Moscow counts on Amman to put pressure on mainstream FSA groups that operate in southern Syria to maintain the truce while Jordan lobbies Russia to put pressure on the Syrian army not to wreck the de-escalation zone.

Israel, Jordan and Washington have also pressed Moscow to remove Iranian-backed forces from areas along the southwest border as part of the ceasefire in southern Syria.