“In the last 48 hours, more than 90 raids have been launched against the residential part of the town,” Syrian Observatory for Human Rights chief Rami Abdel Rahman said.

“The strikes are the most intense since Palmyra was taken by ISIS on May 21,” he said, adding that at least five civilians were killed.

“Dozens of families have fled the town and headed to Raqa, Deir Ezzor and other areas under ISIS control in the Syrian desert,” he added.

A Syrian activist originally from Palmyra confirmed “an intensification in the air strikes on residential areas, with more than 60 raids in 24 hours”.

“It’s difficult for civilians to leave town though, because Palmyra is in the middle of the desert,” added the activist, who uses the pseudonym Mohammed Hassan al-Homsi.

Raqa, in northern Syria and ISIS’s de facto capital in the country, “is the closest (non-regime-held) city and it takes four hours by car,” he said.

“When there are raids, people flee into the desert, even though they are fasting for (the Muslim holy month of) Ramadan and the heat is overwhelming.”

ISIS forces captured Palmyra from the regime after an advance into central Homs province from their strongholds in neighbouring Raqa and Deir Ezzor provinces.

The group’s capture of the town raised international concerns about whether the jihadists would damage Palmyra’s famed ruins.

Thus far, its fighters have not harmed the monuments, and Abdel Rahman said they were also not affected by the regime bombing.