Syrian President Bashar al-Assad has said a coalition between Syria, Russia, Iran and Iraq must succeed “or else the whole region will be destroyed”.
Mr Assad also criticised the US-led coalition and its air strikes in Syria and Iraq as counter-productive, saying that terrorism had only spread.
Meanwhile, Russia has carried out more air strikes in Syria it says targeted so-called Islamic State (IS) positions.
Syrian activists say the raids appear to have focused on other rebel groups.
In an interview with Iranian state television, reported by the Syrian presidency Twitter feed, President Assad said Syria, Russia, Iran and Iraq were united in battling terrorism and would achieve “practical results”, unlike the US-led coalition.
Mr Assad’s international opponents say a negotiated solution to Syria’s four-year-old civil war must involve the president stepping down, although some Western nations now say he could remain during a transitional period.
But Mr Assad insisted: “Discussion about the political system or officials in Syria is an internal Syrian affair.”
Russian air strikes – in depth
Russia’s defence ministry said on Sunday that its aircraft had bombed 10 IS targets in Syria over the past 24 hours – including command posts, a training camp, ammunition stores and a workshop making explosive devices including suicide belts.
It said its air campaign, which began on Wednesday, was being expanded.
“As a result of our air strikes on Isil [IS] targets, we have managed to disrupt their control system, the terrorist organisation’s supply lines, and also caused significant damage to the infrastructure used to prepare acts of terror,” the ministry added.
Syrian activists said Russian strikes in central Homs province had killed at least two children and a shepherd, and wounded a further 15 people.
The Syrian authorities have reportedly detained a prominent opposition figure, days after he criticised the Russian air strikes.
Munzer Khaddam, spokesman for the National Co-ordination Committee for Democratic Change, was held at a checkpoint near the capital Damascus, an official from his group told AFP news agency.
Turkey and Britain have condemned Russia’s military intervention in support of President Assad.
Turkish President Recep Tayyip Erdogan described it as a grave mistake that would further isolate Moscow.
In London, Prime Minister David Cameron said Russia’s actions would lead to further radicalisation and terrorism.
“Tragically, what has happened is that most of the Russian air strikes, as far as we have been able to see so far, have been in parts of Syria not controlled by Isil (IS) but controlled by other opponents to the regime.
“So what is happening is that they are backing the butcher Assad, which is a terrible mistake for them and for the world; it’s going to make the region more unstable, it will lead to further radicalisation and increased terrorism.
“And I would say to them: change direction, join us in attacking Isil but recognise that if we want to have a secure region, we need an alternative leader to Assad. He can’t unite the Syrian people.”
The US-led coalition carried out 16 air strikes against IS militants, weapons and buildings in Syria and Iraq on Saturday, according to a US military statement.
Syria’s civil war
Why is there a war in Syria?
Anti-government protests developed into a civil war that four years on has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and Jabhat al-Nusra, as well as less numerous so-called “moderate” rebel groups, who are strongest in the north and east. These groups are also battling each other.
What’s the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad – including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon’s Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.