BEIRUT/HANGZHOU, China: Syrian government forces and their allies again laid siege to rebel-held eastern Aleppo Sunday, while Turkish-backed fighters drove Daesh (ISIS) from all the areas along its border, in two significant but separate developments in the multisided conflict.

The fighting – two potential turning points in the conflict if the gains can be sustained – complicated efforts by the United States and Russia to reach a cease-fire deal for Syria.

Talks by the Cold War foes on a cease-fire were set to continue Monday, but “we’re not there yet,” U.S. President Barack Obama told reporters at the G-20 summit in the Chinese city of Hangzhou.

On Sunday, intense aerial and artillery attacks helped government forces and their allies drive insurgents out of the Ramouseh military complex in Aleppo, according to rebels and Britain-based Syrian Observatory for Human Rights.

The rebels had captured the complex in early August, breaking through a government siege of eastern Aleppo. Sunday’s government advances resulted in a new siege of the area, said Zakaria Malahifji of the Fastaqim rebel group.

Russian support has turned the war in Assad’s favor in many areas, although rebels have made some gains, including in Hama province, further south.

Rebels launched a campaign Sunday to try to capture the town of Muan, north of the city of Hama, the provincial capital, said Mohammad Rasheed, a spokesman for rebel group Jaish al-Nasr. Advances by the insurgents in recent days have brought them to within 10 km of government-controlled Hama, the Observatory and rebels say. In a separate battle further east, rebels backed by Turkey – and made up of Aleppo-based factions – drove Daesh militants from all areas they controlled along the Syrian-Turkish border, according to the rebels, Ankara and the Observatory.

Some 10 days ago, Turkey mounted its first full-scale incursion into Syrian territory since the conflict began in 2011, aimed at Daesh and at U.S.-backed Kurdish forces in the area, which have also been battling the militants.

The Turkish-backed advance denied Daesh its main route to the outside world, through which it has moved fighters and weapons.

With support from Turkish tanks and warplanes, the rebels now appear to have secured a roughly 90-km stretch of land that Turkey long wanted to control to keep out militants and to stop the advance of the YPG.

Sunday’s advances illustrate the complexity of the Syrian conflict, which has drawn in most world and regional powers. Efforts to end the fighting have been repeatedly confounded.

Even before Sunday’s battlefield developments, U.S. President Obama said that the two countries were struggling to reach a new cease-fire agreement between Damascus and rebels. “We’re not there yet,” Obama told said. “We have grave differences with the Russians in terms of both the parties we support but also the process that is required to bring about peace in Syria.”

An agreement that would stop the fighting and allow more humanitarian deliveries had looked set to be announced by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov in Hangzhou.

Two lecterns had been set up in a room for a news conference. But Kerry emerged alone to say a couple of issues still needed to be resolved and the two sides would resume talks Monday. He did not elaborate.

Officials from the United States and Russia have been meeting since Kerry traveled to Moscow in July with a proposal that would halt the fighting.

A letter from Washington’s Syria envoy, Michael Ratney, to the armed opposition, a copy of which was seen by Reuters, laid out some of the cease-fire terms. It would oblige Russia to prevent warplanes from bombing areas held by mainstream opposition, require the withdrawal of Damascus’ forces from a supply route north of Aleppo, and focus on delivery of humanitarian aid unhindered by warring sides to the city’s population, said the letter, dated Sept. 3.

In return, the United States would coordinate with Russia in fighting against Al-Qaeda, it said, without elaborating.

The Syrian government and Russia were also to avoid bombing areas where more moderate insurgent groups are operating close to Jabhat Fatah al-Sham, previously the Al-Qaeda-affiliated Nusra Front.

Kerry said he would not rush into any agreement just to see it fail again. A senior State Department official, who declined to be named, said Russia had walked back on some of issues that the sides had already agreed on, which was why both sides needed to continue talking.