Islamic State group fighters advanced Friday to the outskirts of Syria's second city Aleppo, despite 10 days of Russian air strikes that Moscow says are aimed at routing the jihadists.

Moscow announced on Friday that its raids had killed several hundred IS fighters and hit more than 60 "terrorist targets" in Syria over the past 24 hours.

Deputy head of the Russian General Staff Lieutenant General Igor Makushev told reporters that "Su-34M and SU-24SM warplanes hit 60 terrorist targets".

He said Russia had bombed a command post in IS stronghold Raqa, killing two senior field commanders and some 200 fighters, according to intercepted radio communications.

Strikes on Aleppo killed "some 100 militants", and other raids struck command posts and training camps in Latakia, Hama and Idlib.

Western governments say the vast majority of Russian strikes have targeted rebel groups other than IS in an attempt to defend President Bashar al-Assad's rule.

And despite the Russian raids, IS militants have reached their closest position yet to Aleppo in northern Syria, a monitoring group reported.

"Dozens of combatants were killed on both sides" as IS drove out rebels from nearby localities as well as a military base, said Rami Abdel Rahman of the Syrian Observatory for Human Rights.

The jihadists are now just over 10 kilometres (six miles) from the northern edges of Aleppo city and three kilometres (two miles) from pro-regime forces positioned at the Sheikh Najjar industrial zone.

"IS has never been so close to the city of Aleppo, and this is its biggest advance towards" the country's pre-war commercial capital, Abdel Rahman said.

- Iranian general killed -

Control of Aleppo is divided between rebel groups in the east and government forces, bolstered by pro-regime militias, in the west.

IS has not had a presence in the city, but the jihadist group on Friday boasted it had "reached the gates of Aleppo".

"IS announced several times that it would launch an offensive on Aleppo without doing it. They were waiting for the right moment and took advance of Russian strikes on other rebels to advance," said jihadism analyst Romain Caillet.

Thomas Pierret, an expert on Islam in Syria, said the US-led coalition bombing IS in Syria was "not very active" in Aleppo, and that Russia's strikes there had hit mostly rebels, allowing IS to push forward.

Iran's Revolutionary Guards announced one of its senior commanders was killed by IS in the Aleppo area on Thursday, without giving the exact circumstances.

A Guards statement said General Hossein Hamedani had played an "important role... reinforcing the front of Islamic resistance against the terrorists" but was killed "during an advisory mission".

The Observatory said Hamedani was the highest-ranking Iranian casualty of the Syrian conflict.

It said he was killed near Kweyris, a military airport in Aleppo province under siege by IS.

Shiite-dominated Iran is a staunch ally of Assad, sending Guards forces and military advisers to aid him against Sunni rebels seeking his overthrow.

Lebanese Shiite group Hezbollah has done much of the fighting to prop up Assad's army, though the commander of the Revolutionary Guards foreign wing, Qassem Soleimani, is said to be heavily involved in strategic planning.

- Moscow denies missile crash -

French Defence Minister Jean-Yves Le Drian said Friday that "80 to 90 percent" of Russian strikes in Syria were aimed at propping up Assad.

He was speaking after French warplanes carried out new strikes overnight on IS targets in Syria, where Paris launched its first raids on September 27.

According to the Observatory, 16 IS jihadists including three child soldiers were killed in the raids which hit "a training camp" on the southern edges of Raqa city.

The Russian air war has provided cover for Assad's ground troops, who have lost swathes of the country to jihadists and rebel groups since 2011.

The campaign has been critical for the regime's fight in Sahl al-Ghab, a strategic plain in Hama province bordering both the regime's coastal bastion of Latakia and the rebel stronghold of Idlib province.

Syria's army announced a "vast offensive" on Thursday, advancing near Sahl al-Ghab from both the Hama and Latakia fronts with Russian air support.

Moscow denied a US claim that four Syria-bound Russian cruise missiles fired from the Caspian Sea had crashed in Iran on Wednesday.

"Any professional knows that during these operations we always fix the target before and after impact. All our cruise missiles hit their target," spokesman General Igor Konashenkov said.

Iran declined to confirm the claim by a US official, who did not provide details about where the missiles might have come down or if they caused any damage.