Acknowledging an unfolding disaster in Syria’s besieged city of Aleppo, where roughly two million people are trapped, Russia conceded on Monday that daily three-hour halts in fighting that it had proposed last week were insufficient, possibly opening the door to longer pauses.

Russia also suggested that it was close to an agreement on a military collaboration with the United States to attack Islamic State fighters in the Aleppo area as part of a solution. Such a joint effort would be a new level of cooperation between the two powers in seeking a way out of the five-year-old Syria war, in which the Russians and Americans basically back opposite sides.

The developments suggested that Russia wished to avoid the appearance of responsibility for the suffering in Aleppo, the once-thriving commercial epicenter of northern Syria that has been a strategic battleground for much of the war.

A week ago, insurgents embedded in the eastern part of Aleppo broke a stalemate against Syrian military forces and their Russian allies, effectively blocking government-held western Aleppo and isolating the entire city.