Istanbul — Syrian rebels firing on government-held areas of Aleppo killed at least 11 people Tuesday, activists and Syrian state media said, in attacks that included a deadly rocket strike on a hospital even as international envoys struggled to find ways to quell the battles.

The U.N. envoy for Syria, Staffan de Mistura, held meetings in Moscow — a key Syrian ally — as part of crisis talks seeking to halt the escalation of violence in Aleppo. More than 250 people have been killed in the past week, monitoring groups estimate.

The official Syrian Arab News Agency said “scores” were killed or injured in rebel shelling on government neighborhoods in western Aleppo. The Britain-based Syrian Observatory for Human Rights, a monitoring group, also said the hospital was heavily damaged.

The number of dead and injured in the attack could not be immediately confirmed.

In Moscow, de Mistura said he hoped to get a so-called “cessation of hostilities” back on track after a week of carnage all but collapsed the U.S.- and Russian-backed cease-fire in Syria.

Both Syrian forces and rebel factions opposing President Bashar Assad have stepped up offensives in the past week around Aleppo, the country’s largest city.

The mounting death toll in Aleppo has stirred international outrage and concern that Syria’s civil war could be tumbling into a new round of bloodshed and humanitarian misery.

The Aleppo attacks have included an airstrike last week on a hospital affiliated with the aid group Doctors Without Borders and the International Committee of the Red Cross.

At least 14 medical staff and patients were killed in the strike, which has been widely blamed on Syrian warplanes.

On Tuesday, the Syrian military said it was battling a wider offensive by “terrorist groups” in Aleppo, the Associated Press reported, citing a military statement.

Last week, the Syrian government announced a “regime of calm” in the capital, Damascus, and in northern Latakia province. U.S. diplomats have urged Syrian authorities — and their Russian backers — to extend the temporary cease-fire to Aleppo.

Russia’s foreign minister, Sergei Lavrov, said in Moscow that the United States and Russia would establish a joint center in Geneva to monitor the Syrian conflict. He did not offer any more details.

“There are groups in Syria trying to escalate violence … and they shouldn’t be allowed to do so,” Lavrov said in a news conference with de Mistura, who met on Monday with Secretary of State John F. Kerry in Geneva.