Islamic State fighters have reportedly captured a town in Homs province from government forces, part of a wider counter-attack by the jihadists as they come under intense pressure in eastern Syria.
There was no comment from the Syrian army on the Syrian Observatory for Human Rights report that the town of al-Qaryatayn had fallen.
The town is located some 300km from Deir al-Zor city, the current focus of the Syrian government's campaign against Islamic State.
Syrian government forces and allied militia recovered Qaryatayn from Islamic State six months ago.
The Observatory said the jihadists had captured it in a surprise attack launched after their fighters had infiltrated the town.
IS has lost swathes of territory to the Russian-backed Syrian government and to US-backed Syrian militias that are waging separate campaigns against the group's last major strongholds in Deir al-Zor province in eastern Syria.
Islamic State fighters last week counter-attacked government forces and allied Iran-backed militias along the highway connecting Deir al-Zor city to Palmyra in central Syria, temporarily rendering the road impassable.
Hezbollah leader Sayyed Hassan Nasrallah, whose group is fighting in support of the Syrian government, said on Sunday (NZT) that IS counter-attacks were to be expected as the jihadists were besieged and incapable of recovering ground.
The Syrian air force conducted intensive air strikes on Islamic State positions and fortifications in several locations, a Syrian military source said in a report circulated by state media on Sunday. It did not mention Qaryatayn.
The source said dozens of IS militants had been killed in the air strikes to the east of the town of al-Salamiya; to the east of the town of al-Sukhna, which is on a main road to Deir al-Zor; and in Deir al-Zor province itself.