On Saturday (3 September) an unnamed US State Department official told Associated Press that a deal between Russian Federation and the US had been reached.
Kerry said at the G20 economic summit in China that the talks have stalled over some "technical" issues but they would resume Monday.
U.S. President Barack Obama has said his country is working with Russian Federation on finalizing a ceasefire deal in Syria.
Even before Sunday's battlefield developments, U.S. President Obama said that the two countries were struggling to reach a new ceasefire agreement between Damascus and rebels.
"It's fair to say that out of the review I think there are a couple of tough issues that we talked about today", Kerry said, declining to give details.
Beyond Aleppo, Russia and the Syrian government of Bashar al-Assad have continued to refuse to allow United Nations humanitarian aid convoys to enter more than a dozen besieged towns and cities held by various rebel forces.
The Syrian civil war has left over 250,000 killed and some 11 million displaced, leading to a refugee crisis in Europe, as well as an escalation in terrorist activities all over the world.
Instead, US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov are set to meet again on Monday in Hangzhou, China, where G20 leaders are gathered.
Erdogan, meanwhile, said his government was preparing to submit additional evidence about the coup to the United States, and would soon send his justice and Interior ministers for consultations with American authorities.
In return, the United States would coordinate with Russian Federation against al Qaeda, the letter said, without elaborating.
In return, the United States would coordinate with Russian Federation in fighting against al Qaeda, it said, without elaborating.
Another problem has been identifying which areas are controlled by the al-Qaeda linked group Nusra Front, now known as Jabhat Fatah Al-Sham, and which by the moderate opposition.
While Front-led forces last month broke through government encirclement from the south to join with opposition rebels occupying the eastern half of the city, that route now also appears to be closed as a conduit for aid as the government launched a major new push to retake it. A senior State Department official, who declined to be named, said Russian Federation had walked back on some of issues that the sides had already agreed on, which was why both sides needed to continue talking.
"We're not going to rush", he said, stressing the importance of reaching a deal that was able "to try to get the job done". Russia, for its part, has said it would only agree to the accord if the USA could separate Syria Conquest Front fighters from rebel forces on the ground, who have grown increasingly intertwined, especially in Aleppo.
Only after the Syrians and Russians stand down and aid starts flowing would the US consider working with the Russians to target the Nusra Front, al Qaeda's one-time affiliate in the country.
The US president said talks on the situation in Syria are hard, because trying to corral all forces on the ground into a coherent structure for negotiations is hard.
Syrian President Bashar al-Assad wants to fully recapture divided Aleppo, Syria's largest city before the war.
Later on Sunday, Obama met Turkish President Recep Tayyip Erdogan, a prickly leader who remains nonetheless indispensable to the United States.
In a separate battle further east, rebels backed by Turkey - and made up of Aleppo-based factions - drove Islamic State militants from all areas they controlled along the Syrian-Turkish border, according to the rebels, Ankara and the Observatory.
"Many of the groups considered acceptable by the U.S. have actually affiliated with the Nusra Front, while the Nusra Front is using them to avoid being attacked", Ryabkov told Russian media, citing a longstanding complaint of his government.