The United Nations mediator on Syria, Staffan de Mistura, said on Tuesday that moribund peace talks on the conflict could be resumed if a faltering truce was extended to the northern city of Aleppo, something he and the Russian foreign minister said might happen within hours.

An escalation of violence in the divided city has undermined efforts to end the Syria crisis.

De Mistura made his upbeat comments after holding talks with Russian Foreign Minister Sergei Lavrov in Moscow, saying he thought there was now a chance to relaunch the cessation of hostilities by reinforcing and extending local truces.

“I have a feeling and a hope that we can relaunch this,” De Mistura told a news conference.

“We all hope that … in a few hours we can relaunch the cessation of hostilities. If we can do this, we will be back on the right track,” he said.

But deadly attacks in Aleppo underlined the challenge diplomats face.

Syrian opposition fighters shelled government-held parts of Aleppo, killing at least 12 people, as the army claimed it was repelling a wide offensive by the rebels in the country’s largest city.

Activists, meanwhile, said Assad regime forces were also shelling rebel-held parts of the city, killing two people and wounding several.

Aleppo has been the center of violence over the past 12 days that left more than 250 civilians dead in the contested city.

The city was excluded from a truce declared unilaterally by the Syrian military last week for the capital, Damascus, and its suburbs and the coastal province of Latakia.

Also in northern Syria, warplanes carried out intense airstrikes on the city of Raqqa, the de facto capital of ISIS, in the early hours Tuesday. Activist groups said it was not clear if the warplanes were Russian or those of the U.S.-led coalition.

The Syrian Observatory for Human Rights, which has a network of activists around the country, said there were more than 35 air raids and that 18 people were killed, including five ISIS members. It said dozens were wounded.