Senior envoys from world and regional powers are to meet once again in Vienna on Tuesday in an effort to salvage stumbling efforts to halt Syria's bloody civil war.

US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov -- diplomacy's odd couple, at loggerheads over much else in the world -- will chair the meeting.

At the gathering, the 17-nation International Syria Support Group (ISSG) will renew its call for a national cessation of hostilities and immediate humanitarian access to besieged communities.

But the third plank of the plan -- a call for Syrian President Bashar al-Assad's regime and the rebels to agree on a "framework for political transition" -- may cause headaches.

US officials travelling with Kerry say Washington still insists Assad should go, with an August 1 deadline for settling on the framework under which he does so.

But the Syrian strongman, bolstered by military support from Russia and Iran, has shown no sign he is prepared to leave and his forces are still battling for territory.

UN-mediated peace talks in Geneva under UN envoy Staffan de Mistura have made little headway and now even Washington's allies privately doubt the August 1 date will be met.

"The target for August is to have a framework in place, a framework agreed, for a political transition," a senior State Department official told reporters in Vienna.

"We're talking to the Russians, trying to get a better environment for the political negotiations," he said. "Particularly on the regime side."

More than 270,000 people have been killed in Syria and millions more driven from their homes since the conflict began with protests against President Bashar al-Assad in 2011. Photo: AFP

The opposition coalition, he said, had been more open about how they would like to see the deal negotiated but Assad's people -- while publicly backing talks -- have not engaged.

"The regime is just not there, and I think that's really the key to it," he said.

This is a key that can only be turned by Russia, if at all. Moscow, Assad's key foreign backer, supports the ISSG platform on paper but backs Syrian government forces on the ground.

INTERNATIONAL LAW

Russia nevertheless endorsed the UN Security Council resolution that enshrined the ISSG peace plan in international law -- and Lavrov says he supports it.

"Today we're a long way from the finish line," he said Monday, according to official Russian news agencies.

"But if everything decided under the auspices of the ISSG and the UN Security Council is put in place honestly, then there's every chance the situation won't stay as it is."

The situation on the ground as it stands is dire.

An AFPTV image shows destruction at a camp for displaced people near the town of Sarmada in Syria's Idblib province, near the border with Turkey, following an air strike on May 5, 2016.

Washington and Moscow chivvied the warring parties into a shaky ceasefire in late February, but pockets of violence remain.

Meanwhile, regime forces continue to blockade several rebel-held areas around Damascus, stopping all humanitarian food and medical aid in defiance of the United Nations.

And jihadist groups the Islamic State and the Al-Nusra Front, not party to the ceasefire or peace talks, still hold vast swathes of the country and carry out attacks.

On Monday, powerful blasts rocked a gas field after what observers said was an Islamic State group attack on the key infrastructure Syria will need if it is to rebuild.

Fighting has also broken out between Al-Nusra and other Islamist factions that chose to take part in the peace process, killing more than 300 fighters in recent weeks.

The Syrian war erupted in early 2011 after Assad's forces launched a brutal crackdown on anti-government protests, and has since killed more than 270,000 people.

Millions have been driven from their homes and a wave of refugees has joined a flow of migrants struggling to escape to Europe, causing a humanitarian and political crisis.

On the ground, US allies like Saudi Arabia and Turkey back some of the rebel factions, while Russia has dispatched war planes and advisors and Iran troops to back Assad.