Islamic State group jihadists killed 15 Syrian soldiers and allied fighters near Palmyra Friday, taking the government's losses in 24 hours of clashes to at least 49, a monitor said.
The jihadists' advance brought them to within four kilometres (two and half miles) of the desert city, which government forces recaptured in March after IS had wrought huge destruction on its famed ancient monuments, the Syrian Observatory for Human Rights said.
IS fighters launched simultaneous attacks on several government positions in the eastern desert of Homs province on Thursday, including areas near the Mahr and Shaar oil and gas fields.
It has seized government checkpoints, silos and the village of Jazal, northwest of Palmyra, in the ongoing assault, the Observatory said.
The army has brought in reinforcements and carried out air strikes against IS in a bid to retake lost territory.
The jihadists too have suffered losses, the Observatory said, although it had no immediate figure.
IS once held several key towns in Homs province, including Palmyra.
It overran the town in May last year, and systematically destroyed temples and tower tombs at the town's UNESCO World Heritage site, before being pushed out by government forces.
It is still present in parts of the east of the province, and has carried out hit-and-run attacks on government positions in recent months.
But Observatory director Rami Abdel Rahman said the current assault was the first to see IS seize territory since it was pushed out of most of the province.
Forces loyal to syria's President Bashar al-Assad walk past damaged buildings in a government held area of Aleppo.