Syrian President Bashar al-Assad has warned the success of Russia's military intervention in his war-torn country is vital for the entire Middle East, with Moscow ramping up its bombing campaign.
Russian raids against what Moscow says are Islamic State group targets took place for a fifth day on Sunday (local time) despite accusations in the West that the strikes are mainly targeting moderate opponents of the regime.
"The alliance between Russia, Syria, Iraq and Iran must succeed or else the whole region will be destroyed," Assad said in an interview broadcast by Iranian state television.
"The chances of success for this coalition are great and not insignificant," he added.
Russia said it had dropped concrete-buster munitions on new IS targets and destroyed command posts, storehouses and other infrastructure.
"From the airbase of Hmeimim, the Russian aviation group is continuing to ramp up air strikes using high-precision missiles against the ISIS facilities in Syria," said the defence ministry, referring to IS.
Western and Gulf countries insist Assad must step down after presiding over more than four years of a civil war that has killed more than 240,000 people.
"If the solution was me stepping down I would not hesitate," a defiant Assad said in the interview.
Washington accuses Russia of seeking to buttress Assad and of targeting Western-backed moderate opposition and IS fighters alike.
British Prime Minister David Cameron urged Russian President Vladimir Putin on Sunday to "change direction" in Syria and recognise that Assad must be replaced.
Putin's spokesman challenged the West's distinction between jihadist and other Islamist rebel groups.
Putin, who met the leaders of France and Germany in Paris on Friday, had "expressed a lively interest in the subject and asked what the difference between the moderate opposition and the immoderate opposition is", Dmitry Peskov said on television late on Saturday.
"So far, no one really has managed to explain what the moderate opposition is."
Moscow is keen to turn the tables on the United States, suggesting it is Washington and its allies that often hit the wrong targets.