Air strikes in northwestern Syria on Thursday, thought to have been carried out by Russia, killed 51 civilians including nine children, according to a Britain-based activist group.

The raids, which hit a neighbourhood in the area of Zardana town in the Idlib countryside, also wounded 50 people, the Syrian Observatory for Human Rights said.

According to activists in Zardana, four more people are missing but rescue workers have ceased their search for them.

"War planes, which are likely Russian, targeted the village of Zardana in northern rural Idlib overnight and caused the highest death toll in a single attack on the region since late March," said Rami Abdulrahman, the Observatory's director.

Omar al-Hanoubi, a media activist in Zardana, told Middle East Eye the strikes came as residents prayed at a local mosque just 50 metres from the blast site. He said three to four missiles were fired to begin with.

"Civilians and paramedics went to rescue people but they were surprised by another strike on the same location. One paramedic was killed and five injured," he said.

"That's why numbers of people killed are high, because they were rescuing while the strike happened."

The Russian and Syrian government air forces have often been accused of such repetitive attacks, known as "double-tap" strikes, which are notorious for killing rescue workers as they attempt to retrieve victims from the site of the original blast.

According to Hanoubi, the blasts left a crater five metres deep. He said 11 buildings were demolished, one of them four floors high.

Zardana, a mainly residential town, has been to a large extent spared fierce bombardment in recent months.

It is largely controlled by Islamist rebels, with a small presence of the Hayat Tahrir al-Sham alliance led by Syria's former al-Qaeda affiliate.

Immediately following the strike there were rumours that militants were present in the mosque, but Hanoubi told MEE this was not true.

The activist said two families who belonged to armed factions were clashing in the neighbourhood when the strike hit, "but the strike were not against them but against the civilians".

He said the majority of victims were women and children.

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Idlib remains the largest populated area of Syria out of the government's control.

In recent years, tens of thousands of fighters and civilians have fled there from parts of the country which the army has recaptured with the help of Russia and Iran.

Since Russia intervened on its side in 2015, the Syrian government has regained control of around half of the country.

More than 350,000 people have been killed in Syria's war since it started in 2011 with the brutal repression of anti-government protests.