A terrifying torture chamber where Syrians were detained and punished by Islamic State fighters has been revealed, reports Daily Mail.

Many local people were held inside the ISIS ‘prison’ building and tortured before being executed in Manbij, a city in northern Syria, a BBC report has found.

Inside the prison, forms questioning local people's knowledge of Islamic rules were left littered on the floor.

The people were graded from 0-100, with a mark of 0-50 classed as 'weak', the BBC's Jiyar Gol reported.

Each tiny cockroach-infested cell reportedly held up to 10-15 people. Cruel isolation chambers that barely contained enough room to stand in were also discovered.

The video report from Manbij comes as at least 20 civilians were killed on Monday in air strikes by U.S.-led coalition planes on the Islamic State-held city.

The UK-based Syrian Observatory for Human Rights said the raids hit the northern Hazawneh quarter of the besieged city where U.S.-backed Syria Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, are engaged in street fighting with the militants.

The monitor said more than 100 civilians had now been killed in U.S.-led raids on the city and its outskirts since the SDF launched a major offensive at the end of May to seize the last territory held by Islamic State on the frontier with Turkey.

Progress into the city has been slow with the militants deploying snipers, planting mines and preventing civilians from leaving, hampering efforts to bomb the city without causing large casualties, Kurdish sources said.

The sources say Islamic State has prevented thousands of the city's more than 50,000 population from leaving, effectively holding them hostage to slow the advance of the SDF fighters.

Rebels and many residents say Russia's bombing campaign has been even more indiscriminate and accuse the Russians of deliberately hitting hospitals, schools and infrastructure in opposition-held areas, something Moscow denies.