UN mediator Staffan de Mistura said Tuesday he hopes that stalled Syrian peace talks can resume in July, but only if the security and humanitarian situation on the ground shows clear improvement.

The UN-backed talks aim to reach a political settlement to Syria's brutal five-year war, but the process has deadlocked while on the ground a fragile truce hangs by a thread.

"The window of opportunity is coming quickly to a close unless we maintain alive the cessation of hostilities, we increase humanitarian aid and we come to some common understanding of a political transition," de Mistura told the General Assembly via video link from Geneva.

"Then we can have, hopefully in July, inter-Syrian talks that are not about principles but about concrete steps to a political transition," he said. "I will consider that in July but not yet, not now because it is premature in view of the current discussions and current situation."

The United States and Russia, co-chairs of the 22-nation International Syria Support Group (ISSG) steering the peace process, have set a target of August 1 to begin substantive talks between President Bashar al-Assad and non-jihadist Syrian rebels on a political transition.

But Mistura said competing views on how a transition could occur were impeding the peace process, with the Syrian regime, the opposition, the United States and Russia all at odds -- particularly over Assad's future role, a main stumbling block in the talks.

"Political talks cannot proceed," he said, "while hostilities are escalating and civilians are starving."

Though some besieged communities have received aid shipments recently, the UN is far from having unobstructed, safe access to Syria's civilian population, de Mistura said.

Since the beginning of the year UN agencies have been able to send land convoys delivering life-saving supplies to 16 of the 18 UN-identified areas under siege -- but those same deliveries are often accompanied by an uptick in violence.

De Mistura also noted a "worrisome escalation of fighting" between Assad's forces and opposition fighters in areas like Aleppo and the Damascus suburbs. 

Also addressing the meeting, the head of UN humanitarian operations Stephen O'Brien urged member states to step up aid contributions. 

He admonished the Syrian government for blocking aid convoys and besieging its own population.

"The barbaric use of medieval siege tactics is morally reprehensible and has no place in the 21st century," O'Brien said.

The UN official accused the regime of removing UN convoys delivering hundreds of thousands of essential drugs and medical equipment.

"Medicine and other relief must not be turned into a cynical political bargaining chip," he said.

Source: AFP