Syrian troops have crossed into Raqqa province, nearing the capital of so-called Islamic State.
The UK-based Syrian Observatory for Human Rights said heavy Russian air strikes had helped the offensive.
There is no Syrian or Russian confirmation.
A US-backed alliance of Syrian Kurdish and Arab fighters started a campaign in May to expel IS militants from land north of Raqqa.
IS seized the city in 2013 from rebels opposed to President Bashar al-Assad.
The jihadist group quickly established its headquarters there and began imposing its vision of a state, implementing a strict interpretation of Islamic law.
It proclaimed a “caliphate” in 2014.
The Syrian Observatory said there had been clashes with IS militants on the Athria-Tabqa road as the Syrian troops moved from Hama province in the south-west.
The Syrian military were trying to reach Lake Assad and the main road linking Raqqa to Aleppo.
Tabqa is home to a key airbase taken by IS in August 2014.
To the north of Raqqa, the US-backed Syrian Democratic Forces (SDF) are believed to have deployed about 30,000 fighters as they try to force IS out of the city.
Russia has promised to co-ordinate with the SDF and the US on the offensive.
The alliance, which is dominated by the Kurdish Popular Protection Units (YPG) militia, has emerged as a key ally of the US-led coalition over the past two years, leading the fight against IS on the ground in northern Syria.
US officials estimate that about 3,000 to 5,000 IS fighters are inside Raqqa.