According to the Syrian Observatory for Human Rights, a flurry of airstrikes pounded the headquarters of al-Qaeda’s Nusra Front in the Idlib Province, with 10 strikes hitting the base and the surrounding area and killing at least 37. The Observatory said most of the dead, around 24, were civilian bystanders.

Exactly who was responsible for the attacks was unclear. The Observatory said it was probably Russian warplanes, or possibly the US-backed coalition, both of whom have been operating in the area recently. Russia, however, was quick to deny their involvement, saying they hadn’t conducted such a strike.

The US has reported “two precision strikes” in Idlib a few days ago, but as yet has neither confirmed nor denied involvement in these most recent attacks. Besides Russia and the US coalition, Turkish warplanes or Syrian warplanes are the only ones operating in Syria.

Nusra has found themselves increasingly targeted in recent months, first related to fighting over the city of Aleppo, and then later as a consequence of internal unrest with more moderate Islamist rebel groups in Idlib.