BEIRUT -- Activists say suspected Russian warplanes have bombed the outskirts of Islamic State-held Palmyra, sending smoke rising out of an area that includes a historic castle overlooking the Syrian city's Roman ruins.

An activist in Palmyra who goes by the name Nasser al-Thaer says at least eight airstrikes struck the area of the Islamic-era castle on Monday. He says it was difficult to assess damage because of the ongoing airstrikes.

The Britain-based Syrian Observatory for Human Rights also says suspected Russian airstrikes caused damage in the city.

Palmyra, seized by the IS group in May, is home to world-famous Roman ruins. The Islamic State group has destroyed a number of its renowned sites.