President Vladimir Putin has defended Russia's military operations in Syria, saying the aim is to "stabilise the legitimate authority" of Syrian President Bashar al-Assad.
Mr Putin told Russian state TV Moscow also wanted to "create conditions for a political compromise" in Syria.
He denied that Russian air strikes were hitting moderate opposition groups rather than Islamic State militants.
Syrian forces are said to have made significant advances against rebels.
Government gains in Idlib, Hama and Latakia provinces were on Sunday reported both by Damascus and opposition activists.
The main battlefront is currently close to the key highway that links the capital with other major cities, including Aleppo, and Mr Assad's forces are believed to be seeking to cut off rebels in Idlib.
In the interview with Rossiya One TV broadcast on Sunday, President Putin said Russia's aim was to "stabilise" the government in Damascus.
He stressed that without Moscow's support for President Assad there was a danger that "terrorist groups" could overrun Syria.
Mr Assad's government was currently "under siege", Mr Putin said, adding that militants were "at the edge of Damascus".
The Russian leader also urged other nations to "unite efforts against this evil (terrorism)".
The US-led coalition - which has been carrying out its own air strikes in Syria - earlier said it would not be co-operating with Russia.
Several countries - including the UK and Turkey - have described Russia's support for President Assad as a "mistake".
Russia said on Sunday its aircraft carried out more than 60 missions over Syria in the past 24 hours, and that IS was its main target.
Russia began its strikes on 30 September.