A series of airstrikes in a rebel-held central Syrian town Wednesday killed at least 12 people, including 10 children and two women who were taking cover in an underground shelter, activists said.

Homs-based activist Bebars Al-Talawy said at least eight air raids struck the town of Rastan, one of them hitting a house, destroying it while its residents were taking cover in the shelter.

The Britain-based Syrian Observatory for Human Rights said the raid killed a whole family, including a man, his wife, two sons and four daughters, in addition to his two sisters and their four children. The Observatory said the number may rise because rescue workers were still pulling people from the rubble.

The Local Coordination Committees said Wednesday's air raid killed 12 people, also confirming that the family was hiding in the shelter.

An amateur video posted by activists online showed several bodies, including those of three children, placed on the floor and covered with a sheet.

The video appeared genuine and corresponded to other Associated Press reporting of the events

The airstrikes came a day after a similar attack on Rastan, which was one of the first areas to rise up against President Bashar Assad's government in 2011.

In the northern city of Aleppo, activists and rescue workers said shells and mortars continue to fall on a main highway, the only lifeline for the rebel-held area of the city to the rest of the country, effectively cutting off the area from the outside world for the third straight day.

Bibars Mishal, a volunteer with the Syrian Civil Defense first-responders group, said an information bulletin was issued warning residents against approaching the highway. Mishal said the shelling came from areas controlled by Kurdish forces, and from government areas.

The Observatory also reported the road was blocked a day earlier. The anti-government Facebook page Aleppo Media Center posted pictures of at least one crater from a shell on the asphalt of the empty road. The highway, known as the Castello Road, has been risky to travel on since the rebel-held area of Aleppo became boxed in by rivals, including government forces, Kurdish fighters and Islamic State militants.

"If the Castello remains cut, the city will become besieged," Mishal said.

Aleppo and its surrounding suburbs has been the scene of some of the fiercest fighting in the country in months. Repeated partial cease-fires ended or collapsed there, and the violence in Aleppo also derailed U.N.-brokered peace talks.

Syria's state news agency SANA also said that three people were killed and four injured when rockets fired by rebels hit the residential area of Hamdaniyeh.

Besides cease-fire violations, a drop in humanitarian aid deliveries has also been at the center of diplomatic efforts. U.N. Special Envoy for Syria Staffan de Mistura said Wednesday that said more efforts needed to be exerted to increase access to besieged areas in Syria.

"Humanitarian aid, which used to be zero to besieged places last year, now is close to 250,000 people. Not enough but moving," he told reporters in Vienna.

On Wednesday, a joint convoy for the International Committee for the Red Cross, the UN and the Syrian Arab Red Crescent delivered a 29-truck convoy of food and medicine to some 10,000 residents in the previously hard-to-reach Damascus suburb of Harasta.

Harasta is the part of rebel-controlled Eastern Ghouta area, which has been besieged since mid-2013. The ICRC said it was the first time it reached the area with aid since 2012.