Russian jets hit Islamic State targets in the Syrian city of Palmyra and the northern province of Aleppo, Syrian state television and a monitoring group said on Tuesday, in some of the heaviest Russian attacks on the hardline Islamist group.
The strikes destroyed 20 vehicles and three weapons depots in Islamic State-held Palmyra, state television said, quoting a military source. In Aleppo, they hit the towns of Al-Bab and Deir Hafer, about 20 km (10 miles) east of a military airport currently besieged by Islamic State fighters.
The Syrian Observatory for Human Rights, a Britain-based group tracking Syria's civil war, said the Palmyra strikes killed 15 Islamic State fighters.
"It was the heaviest Russian attack on Palmyra," Observatory director Rami Abdulrahman said. Across Syria, Russian jets carried out at least 34 air strikes in the last 24 hours, the Observatory said.
Another four Islamic State fighters were killed near Raqqa, the eastern city which has been the group's stronghold in Syria for the last two years.
Al-Manar television, run by the Lebanese Hezbollah group which is allied to President Bashar al-Assad, said Russian planes also carried out four raids in the Jabal al-Zawiya area in the northwestern province of Idlib.
Islamic State forces captured Palmyra in May, an advance which brought them closer to the core of government-held territory in western Syria. It also put the city's Roman-era ruins under the militants' control.
Syria's antiquities chief Maamoun Abdulkarim said on Sunday Islamic State fighters blew up Palmyra's Arch of Triumph, one of the most treasured monuments in the 2,000-year-old city. They had already destroyed temples and other antiquities.
Russia has carried out dozens of strikes in Syria since launching its air campaign last Wednesday.
While Moscow has said its intervention targets the hardline Islamic State fighters who control much of eastern and northern Syria, many of the Russian strikes so far have hit rival insurgent forces opposed to Assad.
Russia said on Saturday it would step up it air strikes in Syria, where a U.S.-led coalition of Western and Arab forces has been waging a year-long air campaign against Islamic State and across the border in neighboring Iraq.
So far, according to Russian defense ministry statements, it has carried out roughly 20 sorties per day.