Ancient ruins in Palmyra
Ancient ruins in PalmyraReuters

Syrian regime forces on Wednesday battled jihadists who cut a key supply route west of ancient Palmyra, after new bombardments hit Aleppo city where a ceasefire was due to expire at midnight.

The latest fighting comes as world powers prepare to meet in Vienna next week to try to revive peace talks aimed at ending a five-year conflict that has killed more than 270,000 people.

The Syrian Observatory for Human Rights, a Britain-based monitor, said the Islamic State (ISIS) group on Tuesday cut the main road from Homs city to Palmyra just weeks after the army recaptured the city, a UNESCO world heritage site.

A military source told the SANA official news agency that the Syrian air force had carried out strikes against ISIS around the main facility in the Shaer gas field, northwest of Palmyra.

A security source told AFP, "Military operations are ongoing in the Shaer gas field," which is one of the biggest in the central province of Homs.

Both sides have been battling each other in the desert around Palmyra since ISIS was ousted from the city in late March.

The jihadist group last week seized the Shaer gas field from the regime.

President Bashar al-Assad's troops retook Palmyra with support from Russian air strikes on March 27 -- an achievement his regime celebrated with concerts in its ancient amphitheater last week.

But ISIS now surrounds Palmyra from all directions except the southwest, Observatory chief Rami Abdel Rahman said, adding that ISIS was within 10 kilometers (around six miles) of the city.

AFP contributed to this report.