A general view of the ancient Roman city of Palmyra, northeast of Damascus, Syria.

Syrian warplanes unleashed a wave of deadly airstrikes on the militant-held town of Palmyra in central Syria on Friday, killing at least 15 and wounding many more, activists said, in some of the heaviest bombardment since the extremist group seized the ancient town May 10.

The Palmyra airstrikes come a day after the Syrian army carried out heavy air raids in the northern city of Raqqa, also held by the Islamic State of Iraq and Syria group (ISIS).

A local activist who spoke on condition of anonymity for security reasons reported at least 30 air raids on Palmyra Friday. A local media collective called the strikes a “massacre” and said 15 people were killed and more than 120 wounded. It said Palmyra’s only hospital was suffering severe shortages in staff and equipment, and some of the wounded had to be taken to Raqqa, more than 200 kilometers (125 miles) away.

It did not say if those killed were civilians or ISIS fighters. However the Britain-based Syrian Observatory for Human Rights said a number of ISIS militants were among those killed.

The Syrian government says it is the leading force fighting ISIS in Syria and Russia, a key ally of President Bashar al-Assad, is trying to convince the West of the need to work with Syria in this effort.

Meanwhile, a coalition of rebel groups launched a major ground offensive on two predominantly Shiite villages in the northern province of Idlib, firing dozens of rockets and detonating at least seven booby-trapped vehicles on their outskirts.

The coalition, known as Jaysh al-Fateh, or “Army of Conquest,” attacked Foua and Kfarya villages earlier Friday. Both are held by pro-government forces in an otherwise rebel-controlled province.

Syrian TV and Manar, a station owned by Lebanon’s Hezbollah group, said popular defense forces – a term used to refer to Shiite militias – foiled attempts by “terrorists” to attack Foua and destroyed five armored vehicles. Hezbollah fighters are also fighting to defend the two villages.

Syrian government forces have pulled out from Idlib province over the past year following major gains by the rebel coalition. The two villages are the only remaining pro-government posts in the region.

The Army of Conquest alliance, which includes Syria’s al-Qaida branch, the Nusra Front, and the extremist Jund al-Aqsa group, is backed by Turkey and Saudi Arabia.

More than 250,000 people have died in Syria’s civil war, according to U.N. officials.