No Syria breakthrough

Syrian government troops renewed the siege of rebel-held parts of Aleppo yesterday, as Washington and Moscow failed to reach a deal on stemming violence in the country's devastating war.

Turkish forces and allied Syrian rebels meanwhile expelled the Islamic State group from the last stretch of the Syrian-Turkish border under its control, a monitor said.

President Barack Obama said earlier that the two sides were "working around the clock" on the sidelines of a summit in China, but that it was "a very complicated piece of business."

The State Department said a deal was close and could be announced by Secretary of State John Kerry and his Russian counterpart Sergei Lavrov, but hours later admitted defeat for now.

"Russians walked back on some of the areas we thought we were agreed on, so we are going back to capitals to consult," a senior State Department official said.

Kerry and Lavrov will meet again today in Hangzhou, where G20 leaders are gathered, he added.

The Secretary of State told reporters: "We're going to review some ideas tonight, a couple things on these couple of tough issues, and come back together and see where we are."

On the ground, Syrian state media said the army and allied forces had taken an area south of Aleppo, severing the sole route left into the eastern neighbourhoods held by the opposition.

The development leaves about 250,000 people living in rebel-controlled parts of the city cut off from the outside world once again, and will raise new fears about a humanitarian crisis in Aleppo.

Meanwhile, the Syrian Observatory for Human Rights monitor said rebels backed by Turkish tanks and warplanes had seized the last parts of the border held by IS from the extremist group, cutting it from the outside world.

"IS has lost its contact with the outside world after losing the remaining border villages," the Britain-based monitor said.

The loss of the Turkish border will deprive IS of a key transit point for recruits and supplies, though the group continues to hold territory in both Syria and Iraq.