Syrian government forces and their allies again laid siege to rebel-held eastern Aleppo on Sunday, while Turkish-backed fighters drove Islamic State 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, whose civil war is in its sixth year.

Talks by the Cold War foes on a cease-fire were set to continue on 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.