Reports say control of the central Syrian city of Palmyra has been going back and forth between government forces and the Islamic State (IS) extremist group.
The British-based Syrian Observatory for Human Rights said on December 11 that Russian air strikes had forced the militants to withdraw to the city's outskirts.
But the monitoring group later reported that IS fighters had again fought their way into the city, forcing government forces to retreat.
More than 4,000 militants have regrouped and launched a second attack to try to recapture Palmyra, Russia's monitoring center in Syria said.
IS fighters entered Palmyra on December 10 after launching an offensive earlier in the week.
The IS group held Palmyra, whose ancient ruins are on the UNESCO World Heritage list, from May 2015 until they were forced out in March of this year.
Based on reporting by Reuters, dpa, Interfax, and the BBC