Air strikes killed 23 people at a holiday spot in Syria's Idlib province while at least 25 died when rebels shelled government-held areas of Aleppo city on Friday, the last day of a 72-hour ceasefire announced by the Syrian army, a war monitor said.

A riverside area in the town of Darkush, near the Turkish border, in western Idlib province was targeted in the air strikes. Idlib province and city are under the control of rebel groups including the al Qaeda-linked Nusra Front.

The dead and injured had come from towns around the province to enjoy the Muslim Eid holiday weekend, witnesses and the Syrian Observatory for Human Rights said.

The death toll, which included 10 women and two children, is likely to rise due to the number of severely injured people, according to the Observatory.

"It was a terrifying sight because most of the people had fallen into the river next to the spring. There were children, women, men," Ahmad Yaziji, a civil defence chief in the nearby town of Jisr al-Shughour, told Reuters.

The civil defence are first responders in opposition-held territory of Syria, now in its sixth year of civil war.

"The area which was targeted had no military positions in it at all and never had done," Yaziji said.

A 72-hour ceasefire was announced by the Syrian army on Wednesday, but rebels and the Observatory said there had been little let-up in the violence.

On Thursday, government forces took a step towards completely encircling rebel-held parts of Aleppo, capturing ground overlooking the only road into the opposition half of the city and effectively putting those areas under siege.

At least 25 people were killed, including six children, in government-held parts of the northern city and more than 120 were injured when dozens of rebel-fired rockets fell on the area on Friday, the Observatory said.

Syrian state media reported roughly similar figures.

Syrian and Russian jets carry out air strikes across Syria but it was not known who carried out Friday's attack in Idlib.

Russia sent warplanes to Syria last year to support President Bashar al-Assad against rebels seeking to end his rule, and have supported Syrian government forces in a separate fight against Islamic State militants further east.

Fighting has intensified since a February ceasefire deal unravelled.-Reuters