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.