But on Saturday ISIS had made new inroads in the city, taking neighborhoods and key sites as Syrian troops focused on a ground operation in the city of Aleppo, the UK-based Syrian Observatory for Human Rights said.
Some Syrian troops have since been diverted to Palmyra as the two sides grapple for control of the city.
ISIS fighters had seized almost the entire city Saturday before Russian warplanes began an intense bombardment, forcing the militants to withdraw to orchards and towns on Palmyra's outskirts.
Russia's Defense Ministry said its jets had delivered 64 airstrikes overnight and claimed to have killed 300 militants in the raid.
"During the night, Syrian government forces, actively supported by the Russian Aerospace Forces, repelled all attacks by terrorists on Palmyra. The attacking side actively used car bombs, armor and rocket artillery systems," the ministry told the state-run Sputnik news agency.
"Eleven battle tanks and infantry fighting vehicles, 31 cars with heavy machine guns, and over 300 militants were destroyed."
ISIS demolished many of the city's ancient treasures, including the 1,800-year-old Arch of Triumph and the nearly 2,000-year-old Temple of Baalshamin, as well as the Temple of Bel. The group beheaded the antiquities expert
who looked after the ruins.
Syria said ISIS also destroyed two Muslim holy sites: a 500-year-old shrine and a tomb where a descendant of the Prophet Mohammed's cousin was reportedly buried.
Palmyra was a caravan oasis when the Romans overtook it in the middle of the first century.
In the centuries that followed, the area "stood at the crossroads of several civilizations," with its art and architecture mixing Greek, Roman and Persian influences, according to UNESCO, the UN agency that documents the world's most important cultural and natural sites.
Eastern Aleppo exodus
US and Russian officials were expected to meet in Geneva on Sunday for more talks about the dire situation in war-ravaged Aleppo. The international community has tried and failed to hammer out a ceasefire for the city, which appears on the brink of falling back into regime control.
The government controls western Aleppo and its troops have made significant territorial gains in the east since its forces entered the enclave by ground on November 27, backed by continual airstrikes. They have now taken around 75% of the area.
Rebel groups held eastern Aleppo for more than four years following the Arab Spring uprising, and a Syrian regime siege on the area had essentially cut it off from the outside world, sparking a humanitarian crisis there.
Now civilians are fleeing by the tens of thousands as the relentless airstrikes leave little hope of survival.
An estimated 10,000 people had fled over the weekend by Sunday afternoon, civilians told CNN -- a number backed by Russian officials. The Syrian state-run news agency SANA put the figure at around 20,000.
CNN Senior International Correspondent Fred Pleitgen witnessed thousands of civilians walking through the southern front lines, many gaunt with fatigue and malnutrition, as the children among them cried in fear.
It is difficult to know how many civilians remain trapped in eastern Aleppo, but 100,000 are estimated to still be living in the enclave. Some are beginning to return to neighborhoods retaken by government forces, faced with the daunting task of rebuilding their homes and communities.
No political solution
A ceasefire for Aleppo has hinged on Russia and the US coming to an agreement. Russia has backed Syrian President Bashar al-Assad's regime with airstrikes since September 2015, targeting all rebel groups that oppose his government. But the US has armed some of those very groups to fight ISIS militants, and Washington and Moscow have sparred in the UN Security Council several times over a political solution to Aleppo.
Russia on Monday vetoed a ceasefire resolution
that would have stopped fighting in Aleppo for at least seven days and allowed desperately needed aid into the east. China also used its veto power, given to all five permanent members of the UN Security Council.
Washington recently agreed to evacuating all rebels out of eastern Aleppo, provided aid would be allowed into the area. The UN has sought assurances from Russia and the Syrian regime that its aid convoys will be given safe passage.
US Secretary of State John Kerry gave a strongly worded speech Saturday after a meeting of foreign ministers in Paris, saying that the Syrian government and Russia must provide guarantees to opposition fighters that they won't be "marching into a massacre" if they are evacuated in a ceasefire.