Russia warned the US-led coalition Wednesday against driving jihadists from Iraq to Syria, in the offensive to recapture Mosul from the Islamic State jihadist group.

"It is necessary not to drive terrorists from one country to the other but to destroy them on the spot," the chief of Russia's General Staff Valery Gerasimov said in a statement.

He said Russia was focusing on "possible attempts by fighters to break out of Mosul" and "freely leave the city in the direction of Syria".

"We hope that our partners in the international coalition realise what freely roaming large IS gangs in the Middle East can lead to."

The long-awaited offensive against Iraq's second city began Monday with air and ground support from a US-led coalition.

Iraqi forces on Wednesday prepared to retake Qaraqosh, the country's largest Christian town located about 15 kilometres (10 miles) southeast of Mosul, a key milestone in their progress toward the jihadists' main hub.

Officials have said the retaking of Mosul -- in Iraq's largest military operation in years -- could take weeks or months and warned that the hundreds of thousands of civilians still in the city could be used as human shields.

Gerasimov said that Russian military satellites, reconnaissance aircraft and drones were surveilling the Iraqi city.

"Despite all the ballyhoo on Western television, it must be understood that the operation as such has not yet begun," Gerasimov said.

Earlier on Wednesday, Russian President Vladimir Putin spoke by telephone with the leaders of Turkey and Iraq to discuss the battle, wishing the Iraqi army success in its operations.

The Kremlin strongman at the weekend called on the US-led coalition to avoid civilian casualties in the battle to recapture the city.

Meanwhile Russian airpower has been backing up an offensive by regime forces on rebel-held eastern Aleppo that has sparked accusations of potential war crimes from the West.

Moscow announced on Tuesday that the Russian and Syrian air forces had stopped bombing Aleppo to pave the way for an eight-hour truce on Thursday.