A frame grab taken from footage from a camera under a plane, released by Russia's Defence Ministry on October 5, shows a Russian military jet dropping bombs during airstrikes carried out by the country's air force near Idlib in Syria.

The Russian defence ministry on Tuesday (Wednesday NZ Time) angrily dismissed reports that its planes had launched air strikes against the Syrian city of Palmyra as false, the TASS news agency reported.

"All reports by foreign media that Russian planes allegedly struck the city of Palmyra are an absolute lie," Major-General Igor Konashenkov, a spokesman for the ministry, was quoted as saying.

"Our planes in Syria do not strike populated areas and especially ones with architectural monuments."

MAXIM SHEMETOV / REUTERS Russian Foreign Ministry spokeswoman Maria Zakharova gestures as she attends a news briefing in Moscow, Russia.

Syrian state television and a monitoring group said earlier on Tuesday (Wednesday NZT) that Russian jets had hit Islamic State targets in Palmyra and the northern provicne of Aleppo, 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 (Monday NZT) Islamic State fighters blew up Palmyra's Arch of Triumph, one of the most treasured monuments in the 2000-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 (Sunday NZT) it would step up it air strikes in Syria, where a US-led coalition of Western and Arab forces has been waging a year-long air campaign against Islamic State and across the border in neighbouring Iraq.

So far, according to Russian defence ministry statements, it has carried out roughly 20 sorties per day.

Meanwhile, the US and its allies carried out 24 air strikes on Islamic State targets in Syria and Iraq on Monday (Tuesday NZT), the Combined Joint Task Force said in a statement on Tuesday (Wednesday NZT).

The latest round of action against the militant group included 17 strikes in Iraq. Nine of them were near Ramadi, hitting tactical units, buildings, fighting positions, bomb-making facilities and other assets. Islamic State positions near Al Huwayja, Habbaniya, Mosul, Sinjar and Sultan Abdulla also were hit.

In Syria, five air strikes were conducted near Al Hasaka and one each near Dayr Az Zawr and Manbij, the task force said.