Hopes that the US and Russia could reach agreement on a joint initiative to tackle Islamic State and stem the violence in Syria’s brutal civil war have suffered a setback after the US said Russia had retreated on some issues it believed had been settled.

The US Department of State had said it was hopeful that the US president, Barack Obama, and Russia’s president, Vladimir Putin, would reach an understanding at the G20 summit taking place in China. But on a day that saw Syrian government troops resume their siege of the city of Aleppo, a planned joint press conference between the US secretary of state, John Kerry, and the Russian foreign minister, Sergei Lavrov, was cancelled. The agreement would have been the culmination of long-running talks between the US and Russia in Moscow, Washington and Geneva.

The deal focuses on a slowdown in the Syrian air campaign against rebel forces in return for an agreement that Russian and Syrian air forces can target Jabhat Fateh al-Sham, the former al-Nusra Front forces linked with al-Qaida. Al-Nusra changed its name to Jabhat Fateh al-Sham (Conquest of Syria) in July in what was seen as a rebranding exercise by some but a genuine ideological shift by others.

Many US-backed Syrian rebel groups have worked alongside al-Nusra and are reluctant to abandon that cooperation, arguing it is an effective fighting force willing to risk lives to defeat the common enemy, President Bashar al-Assad.

In practice, it would be hard to disentangle US-recognised forces from al-Sham and would mean a level of unprecedented intelligence sharing between the US and Russia. Negotiators on both sides have spent weeks looking at maps of areas where opposition groups operate and where Assad’s forces would be prohibited from launching airstrikes. A deal might also open the way for the restoration of UN humanitarian convoys across Syria, and a ceasefire in Aleppo, where 250,000 civilians are still under the threat of starvation in the east of the city.

Speaking on the sidelines of the G20 summit that got under way in Hangzhou, China, on Sunday morning, Obama said both countries still had “grave differences” over the future course of action to put an end to the five years of violence in Syria. “I think it’s premature for us to say there’s a clear path forward, but there’s the possibility at least for us to make some progress,” the US president said.

The UN special envoy on Syria had said he hoped a deal could be struck between in Geneva on Saturday before the G20, at separate meetings in London next week with the main Syrian opposition negotiating committee, and then the UN general assembly in New York later in September. But the talks have not made the expected progress, due partly to the lack of trust between Russia and the US.

The Turkish incursion into northern Syria, part of a bid to force back Kurdish forces supported by the US, has complicated the already multi-layered civil war.

Speaking to the G20, Obama said: “It is a very complicated piece of business. These are difficult negotiations. We have grave differences with the Russians in terms of both the parties we support but also the process that’s required to bring about peace in Syria.

“But if we do not get some buy-in from the Russians on reducing the violence and easing the humanitarian crisis it’s difficult to see how we get to the next phase.”

The US has been sceptical of forging military ties with Russia, blaming the country for violating ceasefire treaties during the Syrian civil war. Russian forces in Syria have also been blamed for attacks on civilian targets, including a field hospital and an airstrike on a Médecins Sans Frontières hospital.

Russia’s deputy foreign minister, Sergei Ryabkov, said a deal was close but that Washington had to dissociate itself from Sham. “Many of the groups considered acceptable by the US have actually affiliated with the Nusra Front, while the Nusra Front is using them to avoid being attacked,” Ryabkov told Russian media.

The situation on the ground near the Turkey-Syria border was developing fast on Sunday, as the Syrian army and allied forces took control of an area south of Aleppo, severing the only remaining route into the eastern neighbourhoods held by the rebels. “The armed forces, in cooperation with their allies, took full control of the military academy zone south of Aleppo and are clearing the remaining terrorists from the area,” state television said, citing a military source.



Meanwhile, the Syrian Observatory for Human Rights said rebels backed by Turkish tanks and warplanes had seized the last parts of the border held by Isis. “Isis has lost its contact with the outside world after losing the remaining border villages,” the UK-based monitor said.



Turkey’s prime minister later confirmed the news. Speaking at a dinner with non-government organisations in the south-eastern city of Diyarbakır, Binali Yıldırım said: “From Azaz to Jarablus, 91km (57 miles) of our border has been completely secured. All the terrorist groups are pushed back – they are gone.”



Much of the work to expel Isis from other parts of the border has been carried out by the Syrian Kurdish YPG, working with the US-led coalition. But Ankara considers the YPG a terrorist group and has been alarmed by its expansion along the border, fearing the creation of a contiguous, semi-autonomous Kurdish region in northern Syria.