Dozens of Islamic State (ISIS) leaders have left their base in Raqqa, Syria, in recent days due to heavy French and Russian bombing and headed to the ISIS-occupied city of Mosul in neighboring Iraq, according to the London-based Syrian Observer of Human Rights.

At least 33 ISIS militants have been killed in the heavy airstrikes of the last three days, according to SOHR activists on the ground in Syria. The death toll is likely to be significantly higher because many of the bodies have been unidentifiable.

The strikes have focused on Raqqa, ISIS' de-facto capital, as well as the nearby towns of Hawad and Al-Meraab.

France's airstrikes are in retaliation for the massacre committed by ISIS militants at six different sites in Paris on Friday night.

Russia has also targeted Raqqa in retaliation for the downing of a Russian airliner in late October that killed 224 people. Russian President Vladimir Putin acknowledged on Wednesday that the plane was downed by a terrorist bomb.

A SOHR spokesman said that at least four ISIS militants – one of them a Briton – had been killed in the strikes, along with an undisclosed number of civilians.