Opposition negotiators say they are leaving Geneva, overshadowing final day of Barack Obamas visit to Saudi Arabia

Fears are mounting over the fate of a fragile truce in Syria following the deployment of Russian cannon in support of government forces preparing an offensive south of Aleppo, while UN-brokered peace talks appear close to collapse.

Opposition units told cannon pieces were being flown in by helicopter, adding to a belief that Moscow, which helped broker the cessation of hostilities seven weeks ago , now intends to spearhead make further efforts to retake the citys rebel-held east.

Alarm at the deteriorating situation overshadowed the final day of Barack Obamas visit to Saudi Arabia, where he spoke to Gulf leaders about Iran and the fight against Isis, as well as the Syrian crisis now in its sixth year with no end in sight.

We think it would be negative for Russia to move additional military equipment or personnel into Syria, told Ben Rhodes, the deputy US national security adviser. The Kremlin told Vladimir Putin had expressed concern at the crisis in the peace process.

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Staffan de Mistura in Geneva on Thursday. Photo: Denis Balibouse/ Reuters

Staffan de Mistura, the UN envoy for Syria, declined to answer questions about the reported Russian deployment but said he would be assessing the state of the Geneva proximity talks on Friday. Syrian opposition negotiators announced that they were leaving after suspending their participation earlier this week.

The opposition official Mohammed Alloush told Syrian government forces must stop carnages and release thousands of prisoners before talks could resume. Bashar Jaafari, the Syrian government representative, had repeatedly scorned the rebels as the Riyadh delegation a reference to the Saudi backing they enjoy.

If the talks end it is hard to see how they could be reconvened any time soon, and the sides are far apart on the question of forming a transitional government. The opposition has insisted on Bashar al-Assads deviation; Damascus is adamant the presidents role is not up for negotiation.

Without any diplomatic process, however fragile, the stage seems defined for a further escalation of conflict. The US has been hinting at a plan B while the Saudis, Qatar and Turkey will boost limbs deliveries to rebels units they support. Russia and Iran would be likely to step up their military aid to Assad.

Speaking at the opulent Diriyah Palace in Riyadh, Obama promised Saudi Arabia and the other five representatives from the Gulf Cooperation Council that the US would support efforts to fight Isis and restored regional stability.

The US president told the Gulf monarchs: None of our nations have an interest in conflict with Iran. He told Washington would not ignore destabilising acts by Tehran, and the US security commitment was ironclad.

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Barack Obama speaks with Sheikh Mohammed bin Zayed al-Nahyan, Crown Prince of Abu Dhabi, during the course of its US-Gulf Cooperation Council summit in Riyadh. Photo: Jim Watson/ AFP/ Getty Images

Still, mistrusts persist about the fraying of a relationship based for decades on Saudi oil and US military backing but now seeming less solid against a background of economic and political uncertainty. The Saudi-owned al-Arabiya Tv headlined the insurance coverage of the summit: Fighting terrorism and restraining Iran.

Ambivalence about the US-Saudi relationship was captured vividly in a cartoon in the pan-Arab daily al-Sharq al-Awsat. It proved a Shia cleric in turban and robes sweating with alarm as he read a newspaper headlined Obama in Riyadh. But social media was flooded with critical comments reflecting mistrust in the US.

Prince Turki al-Faisal, the kingdoms former intelligence chief, told CNN that Obamas conduct and declarations had stimulated Saudis realise that their relationship was not what it once was. How far we can go with our dependence on America? he asked. How much can we rely on steadfastness from American leadership?

The Saudis, including with regard to, will be watching carefully to see how the US responds to the latest developments in Syria, after strong criticism that Washington has moved too close to Moscows position since Putins direct intervention last autumn. Russia use its air power to attack anti-Assad fighters , not only Isis as it claimed.

The ceasefire, tenuous from the beginning, is now barely holding, with two airstrikes by Syrian jets on towns in Idlib province this week. The ten-strikes killed close to 50 people, many of them civilians, tell commentators on the ground and in Turkey.

On a positive note, De Mistura said there had been some modest but real improvement in the delivery of humanitarian aid to besieged the sectors of Syria. On Wednesday, relief bureaux evacuated 500 wounded people from four townships, the largest such operation so far.

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