At least 35 people, including eight children and five rescue workers have been killed in the Syrian city of Aleppo and its outskirts in attacks carried out by the government forces and the rebels, a monitoring group said.

The rebel shelling killed at least 19 people and the government air strikes killed at least 11 on Tuesday, the Syrian Observatory for Human Rights said.

Five civil defence workers - known as "White Helmets" - were also killed by the air strikes and a rocket attack on their centre in a separate incident in the rebel-held town of Atarib, on the outskirts of Aleppo.

The observatory and civil defence officials said the attack appeared to have deliberately targeted them.

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"The targeting was very precise," Radi Saad, a civil defence worker, told the Reuters news agency.

"They were in the centre and ready to respond. When they heard warplanes in the area they did not think they would be the target."

Two people were also seriously wounded and ambulances and cars belonging to doctors were destroyed, another civil defence member, Ahmad Sheikho, said.

It was unclear whether Syrian or Russian warplanes had launched the raids. There was no immediate comment from the Syrian government.

"It is a messy situation," Zouhir Al Shimale, a journalist, told Al Jazeera by telephone from Aleppo.

"From 8am, the aircraft were flying low and the sounds were very loud. They were flying over the [rebel-held area] in Aleppo. There is a feeling among most people that they should stay inside their houses right now."

Several shops and businesses were closed on both sides of the city on Monday and Tuesday, while schools were closed in the rebel-held part of the city, al-Shimale said.

"The schools were closed ... so that they could not be targeted by air strikes."

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The strikes came one day after rebel shelling killed at least 19 civilians and injured 120 in attacks on government-held parts of the city, the observatory and local activists said.

On Monday, the observatory said that at least 60 people were killed between Friday and Sunday in tit-for-tat attacks between government forces and opposition groups in Aleppo.

The Syrian civil war started as a largely unarmed uprising against President Bashar al-Assad in March 2011, but quickly developed into a full-on armed conflict.

United Nations special envoy for Syria, Staffan de Mistura, estimated last week that more than 400,000 Syrians had been killed, though he said that number was not an official UN statistic.

The opposition cited the dire humanitarian situation and ongoing Syrian army attacks when it walked out of negotiations in Geneva last week, saying it needed a "pause". The future of Assad also proved a major sticking point.

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The already shaky ceasefire between the government and some rebels has given way to renewed violence across the country, as government forces carried out air strikes in the Damascus countryside, Homs and other areas.

US President Barack Obama said on Monday that he planned to send 250 more troops to Syria, a sharp increase in the number of Americans working with local Syrian forces.