BEIRUT (AFP) - Islamic State in Iraq and Syria (ISIS) group fighters advanced Friday to the outskirts of Syria's second city Aleppo, despite Moscow's announcement that its raids had killed hundreds of militants in just 24 hours.

With Moscow in its 10th day of strikes, the United States announced it would halt its troubled programme to build up Syrian rebel units to fight ISIS.

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

He said Russia had bombed a command post in ISIS stronghold Raqa, killing two senior commanders and around 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 ISIS in an attempt to defend President Bashar al-Assad's rule.

Pentagon spokesman Peter Cook said Friday Washington would stop recruiting new fighters for its train-and-equip programme and would instead "provide equipment packages and weapons to a select group of vetted leaders and their units" already on the ground.

Despite raids by both Russia and a US-led coalition, ISIS militants have reached their closest position yet to Aleppo city in northern Syria, a monitor reported.

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

The militants are now just over 10km from the northern edges of Aleppo city and 3km from pro-regime forces in the Sheikh Najjar industrial zone.

"ISIS has never been so close to the city of Aleppo, and this is its biggest advance towards" Syria'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.

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

"ISIS 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 Islamic militant analyst Romain Caillet.

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

The coalition headed by Washington said Friday it had conducted one air strike in Aleppo and one on Raqa the previous day.

Meanwhile, Iran's Revolutionary Guards announced one of its senior commanders was killed by IS in the Aleppo area on Thursday.

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 and that he died near Kweyris, a military airport in Aleppo province under siege by ISIS.

Shi'ite-dominated Iran and powerful Lebanese Shi'ite group Hezbollah are both staunch Assad allies.

MOSCOW DENIES MISSILE CRASH

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

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

According to the Observatory, 16 ISIS militants 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 territory to militants 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 its 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.