UN chief Antonio Guterres calls for cessation of military escalation and return to ceasefire agreed by US and Russia.

Syrian government helicopters have pressed on an assault on opposition- held areas of the country’s southwest, dropping barrel bombs in defiance of US demands that President Bashar al-Assad halt the offensive.

The barrel bombs on Friday targeted a cluster of rebel-held towns including Busra al-Harir, northeast of Deraa city, where the attack threatens rebels holding ground jutting northwards into land held by the government.

Syria‘s army has been pushing a week-long assault on the area bordering Jordan and the Israeli-occupied Golan Heights, which Assad has sworn to capture, threatening a “de-escalation” zone agreed upon by the US and Russia last year.

The US on Thursday reiterated its demand that the zone be respected, warning Assad and his Russian allies of “serious repercussions” of violations.

It accused Damascus of initiating air attacks, artillery and rocket attacks.

United Nations Secretary-General Antonio Guterres, on his part, called on Friday for an immediate end to military escalation in southwestern Syria, saying he was “concerned at the significant risks these offensives pose to regional security,” a spokesman for the UN chief said.

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US Ambassador to the United Nations Nikki Haley separately urged Russia to pressure its Syrian ally to uphold the truce.

Haley said in a statement earlier: “The Syrian regime’s violations of the ceasefire in southwest Syria need to stop.”

“We expect Russia to do its part to respect and enforce the ceasefire it helped establish, and to use the influence it has to stop the Syrian regime’s violations and any further destabilizing actions in the southwest and throughout Syria.

“Russia will ultimately bear responsibility for any further escalations in ‎Syria,” Haley said in a statement.

Guterres is scheduled to hold talks on Saturday in Washington, DC, with Secretary of State Mike Pompeo.

Nearly 12,000 people have been displaced in the upsurge of violence that has included air attacks, artillery, barrel bombs and rocket attacks, according to the US statement.

Assad’s forces have retaken large parts of Syria from opposition fighters since Russia intervened militarily on its side in 2015.

A major offensive would risk a wider escalation that could draw the US deeper into the war.

The southwest is of strategic concern to US-allied Israel, which has this year stepped up attacks on Iran-backed armed groups allied to Assad.