Syrian President Bashar al-Assad warned the success of Russia's military intervention in his war-torn country was vital for the entire Middle East, as Moscow ramped up its bombing campaign Sunday.
Russian raids against what Moscow says are Islamic State group targets took place for a fifth day 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.
Washington accuses Russia of seeking to buttress Assad and of targeting Western-backed moderate opposition and IS fighters alike.
US Defence Secretary Ashton Carter arrived in Madrid Sunday on the first leg of a European tour and will meet officials from Spain, Italy and Britain as well as NATO amid Western alarm over Russia's foray into Syria.
President Barack Obama called Russia's dramatic intervention a "recipe for disaster", but pledged Washington would not be drawn into a proxy war.
British Prime Minister David Cameron urged Russian President Vladimir Putin on Sunday to "change direction" in Syria and recognise that Assad must be replaced.
"Tragically, what has happened is that most of the Russian air strikes... have been in parts of Syria not controlled by ISIL (IS) but by other opponents of the regime," Cameron told the BBC.
- 'What's the difference?' -
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 "asked what the difference between the moderate opposition and the immoderate opposition is", Dmitry Peskov said on television late Saturday.
Moscow is keen to turn the tables on the United States, suggesting it is Washington and its allies that often hit the wrong targets.
"When the conversation has turned to this, our president remembered," Peskov said of the Paris talks.
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"He also remembered the wedding in Yemen and so on," Peskov added, referring to an alleged air strike that killed more than 130 people and for which a Saudi-led anti-rebel coalition denied responsibility.
An apparent US air strike on an Afghan hospital that killed 19 people on Saturday is also expected to play into the Kremlin's hands.
Assad on Sunday poured scorn on Western-led efforts to take on IS, which has managed to make some gains in Syria even though it has been bombed for more than a year by US-led coalition jets.
"I haven't seen results. I even see results that are contrary (to the coalition's aim). Terrorism has seen a geographic expansion and the number of recruits to terrorist groups has increased," he said.
- 'Grave mistake' -
Turkish President Recep Tayyip Erdogan, one of Assad's fiercest critics, reiterated his call for Moscow to change its strategy.
"The steps Russia is taking and the bombing campaign in Syria is unacceptable in any way for Turkey," Erdogan said.
"Unfortunately, Russia is making a grave mistake."
German Chancellor Angela Merkel said that while "military efforts" were needed in Syria, they would not be enough to end the bloodshed.
"We need a political process, which has had difficulties getting under way," she said on radio.
Putin has said Moscow needs to hunt down IS militants before they cross into Russia, which has a large Muslim population.
Moscow said on Sunday that over the past 24 hours its jets had made 20 sorties against 10 IS targets.
But the Syrian Observatory for Human Rights, a British-based monitoring group, said that "warplanes, believed to be Russian, carried out many strikes against two villages in the north of Homs province".
One person was killed and others wounded, it said. Another six civilians were killed in apparent Russian strikes in Idlib, the Observatory said on Saturday.
Russia denies its strikes have led to civilian casualties and has instead hailed the results of the first three days of bombing, referring to "panic" among militants.
Raids ordered by Moscow are reported to have hit areas controlled by moderate groups and prompted a claim by US Senator John McCain that Russian jets had killed rebels trained and funded by the CIA.