The Syrian army on Sunday advanced into the northern province of Raqa as part of an offensive to reach the neighbouring oil-rich province of Deir Ezzor, a monitoring group said.
The Britain-based Syrian Observatory for Human Rights said regime troops had reached the edges of Resafa, a town some 40 kilometres (25 miles) southwest of Raqa city, bastion of the Islamic State group.
The goal of the operation is to push further east and reach Deir Ezzor, the Observatory said.
The regime has lost control of many oil and gas fields since the start in 2011 of Syria's conflict, especially in the provinces of Homs and Deir Ezzor.
Sunday's advance came after government troops captured several villages in the past few days, said the Observatory.
Troops were also about 50 kilometres from a US-backed Kurdish-Arab alliance of fighters which launched a new offensive this month to retake Raqa city.
The fighters of the Syrian Democratic Forces entered the northern city earlier in June, after months of heavy clashes to encircle Raqa.
Since June 6 they have seized a few neighbourhoods, including one in the east and one in the west, and are battling to push into the Old City of Raqa.
The Observatory says a large number of IS fighters are holed up in the Old City.
IS seized Raqa in 2014, transforming it into the de facto Syrian capital of its self-declared "caliphate".