Islamic State fighters attacked a military airport in Syria's Homs province on Monday as they pressed a westward offensive against government strongholds, a monitoring group said.
Forays by Islamic State, which is strongest in the northeast and east, into the provinces of Homs, Hama and even Damascus pose a fresh challenge for Syrian President Bashar al-Assad.
Syria's army has seized control of a swathe of territory from Damascus through the cities of Homs and Hama to the western coast by defeating other, less powerful militias including rebels fighting under the banner of the Free Syrian Army.
The Syrian Observatory for Human Rights, which tracks the violence through a network of sources in the country, said Islamic State attacked a military airport in Tadmur, a town in Homs province, early on Monday.
Syrian officials could not immediately be reached for comment, and the fighting was not reported on state media.
Observatory head Rami Abdulrahman said 74 soldiers in Hama province had been killed by Islamic State, which he speculated launched both attacks to raise morale after losses to Kurdish forces in the northeast.
The offensive followed a three-day battle that erupted on Friday further west in Hama around Sheikh Hilal village, the Observatory said. Islamic State was trying to cut the road from Hama to Aleppo, once Syria's most populous city, it added.
A Syrian official said Islamic State had killed around 70 people in the Sheikh Hilal area in what he described as a "massacre against civilians", adding that some of the bodies were mutilated.
Some of those killed may have been from locally formed defense groups or were off-duty soldiers but there were no military casualties, he said.
Around 200,000 people have been killed since 2011 in Syria's civil war, which pits Assad against a range of rebels including jihadist groups such as Islamic State and Al-Qaeda's Nusra Front. A US-led coalition is bombarding Islamic State in both Syria and Iraq.
Kurdish forces, backed by coalition air strikes, defeated Islamic State in the northern Syria town of Kobani this year and other areas in the northeast.
Syrian state television said Syria's army killed 19 Islamic State fighters on Monday in the eastern province of Deir al-Zor, one of the insurgent group's strongholds.
Government supporters posted a video on YouTube on Saturday showing trucks covered in the national flag carrying coffins of people said to have been killed fighting Islamic State in Hama province.
The footage was said to be taken in Salamiyah, a religiously-diverse town east of Hama that has been attacked by jihadist brigades.
An Islamic State fighter told Reuters on condition of anonymity that the Hama campaign aimed to eventually take Salamiyah. "The ultimate goal is liberate Salamiya and Hama but it will not happen before Islamic State is 100 percent ready," he said.
