Forces loyal to President Bashar al-Assad took the towns of Maheen and Hawareen, some 20 kilometres east of the main road connecting the capital Damascus to the north of Syria and the government’s coastal strongholds, the Syrian Observatory for Human Rights said.
The gains bring government forces close to al-Qaryatain in the country’s central desert, the last town on the route to the ancient city of Palmyra which fell to the jihadists in May.
The Observatory said government forces were also advancing on the outskirts of Palmyra, which lies at the heart of the desert separating heavily populated central Syria from Islamic State-controlled eastern areas near the Iraqi border.
State television said the Syrian army command was determined to drive away “the terrorists from the ancient city very soon.”
Since capturing the UNESCO-listed World Heritage Site, Islamic State has destroyed its most famous monuments including ancient temples, funeral towers and a triumphal arch.
The city’s Roman-era theatre and columns lining its main street have been turned into backdrops for the executions of Islamic State prisoners.
The latest advances come nearly two months into a Russian air campaign in support of al-Assad.
They are the second significant gain by Russian-backed Syrian forces at the expense of Islamic State.
Earlier this month troops pushed through jihadist-held territory near Aleppo in northern Syria to relieve an isolated airbase that had been under siege for two years.
Moscow says its air strikes target Islamic State and other “terrorists,” but anti-Assad rebels and their backers say moderates and Islamist opposition fighters not associated with Islamic State have borne the brunt of the attacks.
Government offensives against these Western- and Gulf-backed rebels and their al-Qaeda-linked allies since Russian airstrikes started in central Syria at the end of September have made only limited gains.
Meanwhile, the opposition Syrian National Coalition said Monday that it would attend a meeting due to be held next month in the Saudi capital Riyadh aimed at unifying opposition groups.
“We hope we will come out of the Riyadh conference with results that will expedite the political solution in Syria,” Khaled Khoja, head of the Turkish-backed alliance of political opposition groups, said in Istanbul.
Saudi Arabia last week said it would host a meeting for Syrian opposition forces to unify their ranks in the run-up to planned peace talks with al-Assad’s regime.
Syria’s opposition is plagued by internal divisions. The numerous rebel groups who control territory – most of whom are hardline Islamists – recognize no overarching authority.
Two of the most powerful rebel forces, the hardline Islamist Ahrar al-Sham and Army of Islam, are expected to attend the talks, along with smaller and more moderate groups fighting under the banner of the Free Syrian Army.
Khoja repeated the opposition’s refrain that there could no political solution in Syria as long as “Russian occupation” and al-Assad’s presidency continued.
Russia and Iran have rejected repeated calls by the West and Saudi Arabia for al-Assad to step down to end Syria’s conflict.