By Ellen Francis and Angus McDowall

BEIRUT (Reuters) – Syrian rebels began a counter-attack in Aleppo on Friday with heavy shelling of government-held areas after a weeks-long Russian-backed offensive against besieged districts held by insurgents, rebels said.

Several children were killed at a school in government-held western Aleppo on Thursday, state media and a monitoring group said, just a day after air strikes on a school in a rebel-held village in Idlib, 50km (32 miles) away.

The shells hit two neighborhoods, Syrian state news agency SANA reported: the national school in the Shahaba area, killing three children and wounding more than a dozen others, and the other attack killed three people in Hamdaniya.

The Syrian Observatory for Human Rights, a Britain-based monitor of the five-year-old war, said six children under the age of 16 had been killed in the two attacks, including a baby.

Wednesday’s air strikes hit a school in the Haas village in rebel-held Idlib, killing at least 15 school children, an attack that Western countries have blamed on the Syrian military and Russian air force. Moscow has denied involvement.

 The Syrian Observatory for Human Rights, a British-based group that reports on the war, also said that insurgents had set off several suicide car bombs on the western edge of Aleppo.

A Syrian military source said an insurgent attack in that area had been thwarted. A state TV station said the army had destroyed four car bombs.

The rebels aim to break a siege that government and allied militias imposed this summer with air support from the Russian air force. The rebel-held eastern districts of Aleppo have been subjected to a fierce bombardment since the army declared an offensive to capture the area last month.

“There is a general call-up for anyone who can bear arms,” a senior official in the Levant Front rebel group, which fights under the Free Syrian Army (FSA) banner, told Reuters. “The preparatory shelling started this morning,” he added.

The attack appeared to have been mostly launched by rebel fighters from outside the city against government forces that hold its western districts.

A spokesman for Ahrar al-Sham, a large Islamist rebel group, also said in a social networking message that an offensive on Aleppo had begun on Friday. Factions involved in the attack include Free Syrian Army groups and Jaish al-Fatah, an alliance of Islamist factions, the Levant Front official said.

Grad rockets were launched at Aleppo’s Nairab air base, said Zakaria Malahiji, head of the political office of the Aleppo-based Fastaqim rebel group, adding that it was going to be “a big battle” with all the insurgent groups there participating.

Heavy rebel bombardment, with more than 150 rockets and shells, struck districts on the southwest of the city, the Observatory reported. It said more than 15 civilians had been killed and 100 wounded by rebel shelling of government-held western Aleppo. State media reported that five civilians were killed.

The Observatory also said that Grad surface-to-surface rockets had struck Nairab air base and also locations around the Hmeimim air base, near Latakia.

Syria’s civil war, now in its sixth year, pits President Bashar al-Assad, backed by Russia, Iran and Shi’ite militias from neighboring states, against mostly Sunni rebels including groups supported by Turkey, Gulf monarchies and the United States.

Aleppo, Syria’s most populous city before the war, has for years been split between a government-held western sector and the rebel-held east, which the army and its allies managed to put under siege this summer.

(Additional reporting by Tom Perry, Writing by Angus McDowall, Editing by Angus MacSwan)