Syria conflict: Assad forces make 'significant gains'
Syrian forces backed by Hezbollah militants from Lebanon are said to have made significant advances against rebels after heavy Russian air strikes.
Government gains are being reported in Idlib, Hama and Latakia provinces.
Russia says its aircraft carried out more than 60 missions over Syria in the past 24 hours, and that the Islamic State group was its main target.
But the Russian strikes appear to have impacted heavily on rebels fighting both the government and IS.
The main battlefront is currently close to the key highway that links the capital Damascus with other major cities, including Aleppo, and President Bashar al-Assad's forces are believed to be seeking to cut off rebels in Idlib.
Before Russia's intervention, Idlib had all but fallen to a rebel coalition that had been seriously threatening Mr Assad and his heartland as well as fighting IS, BBC Arab affairs editor Sebastian Usher reports.
The government gains were reported both by Damascus and opposition activists.
Analysis: Jim Muir, BBC News, Beirut
Government forces are basically trying to win back areas they lost earlier this year, to the north of the city of Hama, and in the northern mountains of Latakia province near the coast. Rebels had penetrated there after unifying their ranks and with more concerted backing from their outside supporters, Turkey, Saudi Arabia and Qatar.
That posed a real threat to the heartland of Bashar al-Assad's regime, and it is almost certainly what triggered the Russian intervention and a stepped-up role by Iran.
A senior commander of the al-Nusra Front, the al-Qaeda affiliate which has forces in the area, has issued a call to all the rebel groups to unify and launch a co-ordinated counter-attack on all fronts.
He said if the rebels lost the initiative to the regime and the Russians, they would suffer a series of collapses and their future would be bleak.
Russia says its strikes, which began on 30 September, have been closely co-ordinated with the Syrian government. Russian President Vladimir Putin said on Sunday that the strikes were the execution of "previously drawn plans".
"We persistently conducted reconnaissance, for a long period, from space and from the air," he told the Rossiya 24 news channel.
Russia's defence ministry said: "Su-34, Su-24M and Su-25SM planes carried out 64 sorties from the Hmeymim air base against 63 targets in the provinces of Hama, Latakia, Idlib and Raqqa."
The US-led coalition that has been targeting IS in Syria and Iraq announced it had carried out 24 sorties on Saturday, seven of them in Syria.
Russia said that a second video conference with the US military to discuss ways of avoiding accidents between the two countries' planes over Syria had been "professional and constructive".
In his comments to Rossiya 24, Mr Putin said he believed his country's military intervention in Syria had the support of both Sunni and Shia Muslims across the Middle East.
Questioned about the Middle East's sectarian divisions, the Russian leader said: "In Syria, we do not, under any circumstances, want to get tied up in any inter-confessional conflict."
In another development, the Russian leader held talks in the Russian resort city of Sochi with the Saudi Defence Minister, Mohammed bin Salman.
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.