Updated

Rebels fired a barrage of shells at government-held districts in the northern city of Aleppo Monday, killing at least 23 people, including eight children, and wounding more than 100, state-run TV said.

Syrian TV said the shelling hit the area of Rahman Mosque and other districts in the contested city, shattering balconies and store fronts.

The Britain-based Syrian Observatory for Human Rights said more than 250 shells struck several neighborhoods of Aleppo, killing at least 13 people and wounding scores of others. The group said some people are still missing and the death toll is likely to rise.

Aleppo has seen heavy fighting and shelling since rebels seized part of the city in 2012.

Rebels have targeted government-held neighborhoods in the city in the past, killing dozens, while the Syrian air force has attacked opposition-held parts of Aleppo with barrel bombs, killing hundreds of people.

Syria's crisis, which began in March 2011, has killed more than 220,000 people.

In Damascus, the United Nations' special envoy for Syria arrived in the country Monday to meet with government officials. U.N. spokesman Stephane Dujarric said Monday that Staffan de Mistura planned to emphasize to Syrian officials that the indiscriminate use of barrel bombs was unacceptable.

"Mr. de Mistura intends to raise with the Syrian government the issue of protection of civilians, underlining once again the unacceptable use of barrel bombs and the uncontestable duty of any government in all circumstances to protect its civilians under international humanitarian law," Dujarric said.

The U.N. envoy also intends to discuss ways to implement the June 2012 Geneva communique calling for a transitional government. De Mistura has been meeting with many Syrian groups since May and will continue consultations into July before submitting a report to Secretary-General Ban Ki-moon on implementing the Geneva agreement.

De Mistura "plans to convey his deep conviction that no solution to the Syrian conflict can be imposed by force and that an inclusive and Syrian-led and owned settlement is urgently needed," Dujarric said.

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Associated Press writer Edith M. Lederer at the United Nations contributed to this report.