The Syrian army continues to move into the northern province of Raqqa, home to the de facto capital of the self-claimed caliphate of the Islamic State group fighting in Syria and neighboring Iraq.
The army first pushed into Raqqa Saturday.
The Britain-based Syrian Observatory for Human Rights says Syrian troops have reached the “administrative border” of Raqqa province, backed by Russian airstrikes.
According to the activist group, Syrian troops engaged with IS fighters along the highway that links Raqqa with the Hama province in the southwest.
Syrian troops are hoping to reach Lake Assad and the main road that links Raqqa to Aleppo, another IS stronghold.
IS extremists have held Raqqa city since 2013 when it seized the city from rebels opposing President Bashar al-Assad.
State media in Syria report heavy casualties among the Islamist militants. The Syrian army has yet to release any official statement on the number of dead.
The observatory said Saturday that during three days of fighting at least 26 Islamic State militants had been killed along with nine from the Syrian troops and allied forces.
Syrian troops began to move toward the province Wednesday, the same day U.S.-backed forces attacked the IS-stronghold of Manbij, abput 115 kilometers to the northwest of Raqqa.
Troops with the U.S.-backed, predominantly Kurdish, Syrian Democratic Forces (SDF) advanced closer to Manbij Saturday. The observatory said SDF fighters had captured 34 villages near Manbij.
IS fighters are also engaged in fierce battles with the al-Nusra front for the rebel-held stronghold of Marea, a town nearly 70 kilometers west of Manbij, but so far have not captured the city.
While fighting continues in Raqqa province, Manbij and Marea, IS fighters are also coming under fire in their stronghold of Fallujah by Iraqi government forces that launched their offensive on the city almost two weeks ago.
Violence in and around the contested northern city of Aleppo, Syria's largest and once commercial center, claimed more lives Saturday, according to Russian sources that put the number of killed at 40 and wounded at 100.
The city has been the scene of fierce fighting, even during a cease-fire brokered by the U.S. and Russia that took effect in late February before collapsing weeks later.