Bombing by Syrian government and Russian forces of the last opposition-held pockets of Syria has killed 1,300 people and displaced almost 1 million more since April, according to monitors.

The Syria Campaign, a UK-based charity, said the Syria Civil Defence rescue service reported that 304 children and 11 rescue workers had died in the bombardment across north-west Idlib province and the surrounding countryside.

Hours after the figures were released on Wednesday, attacks by the forces of the Syrian president, Bashar al-Assad, and his Russian allies killed at least 21 civilians, including 10 children, in rebel-held Idlib province.

The Syrian Observatory for Human Rights said a ground-to-ground missile fired by regime forces hit a maternity facility at a makeshift camp for the displaced close to the border with Turkey. The missile killed 15 civilians, including six children, and wounded about 40 others.

Russian military aircraft also targeted the town of Maaret al-Numan in the south of the province, the Observatory said, and “six civilians were killed, among them four children”.

A photographer who works with AFP saw rescue workers retrieve the dust-covered body of a girl from debris and place her in an ambulance.

Assad and his Russian allies began airstrikes and a limited ground campaign against the remains of the Syrian opposition on 26 April.

At least 11,500 airstrikes have been carried out since then, including the documented use of more than 460 cluster bombs and 1,280 barrel bombs, illegal under international law, used to target civilian infrastructure such as schools and hospitals. About 42,000 people have fled their homes in the last week alone.

Ahmad Arafat, the manager of Hurras Child Protection Network in Idlib, said: “With winter approaching, the situation is very bad in the camps. Those who have fled recently are living under trees without shelter or tents to protect them. Children live in constant fear and are unable to play outside or go to the park. Anywhere with groups of people is a possible target for bombing. Schools are particularly dangerous.”

A ceasefire deal brokered by Russia and Turkey in September last year was supposed to save Idlib from the impending regime attack, but it unravelled after the hardline Hayat Tahrir al-Sham (HTS) wrested control of the area from more moderate rebel groups in January.

Aid agencies warned before last year’s ceasefire that an assault on Idlib would put the lives of 3 million civilians in danger and trigger the worst humanitarian disaster of the war to date. Originally home to about 1 million people, Idlib’s population was swollen with civilians displaced by fighting elsewhere in the country.

Despite a supposed ceasefire, fighting also continues in Syria’s north-east, where Turkey launched an attack on western-backed Kurdish-led forces last month.