Syrian news agency SANA reported
that over 4,000 militants swarmed the city from "various directions," despite having suffered heavy losses from bombardments by the Syrian air force. The Russian Defense Ministry had earlier reported that its aircraft had also taken part in the air campaign.
The UK-based Syrian Observatory for Human Rights (SOHR) confirmed that Palmyra had fallen to ISIS on Sunday after Syrian armed forces pulled out from the desert city, the organization said.
"Despite the ongoing air raids, IS retook all of Palmyra after the Syrian army withdrew south of the city," said Observatory head Rami Abdel Rahman, using another acronym for the group.
ISIS' own media wing, the Amaq news agency, also reported that the jihadist group's forces had regained "full control" of the city.
ISIS first seized control of Palmyra, a UNESCO World Heritage Site,
in May 2015. Syrian government forces recaptured it in March this year.
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, SOHR said.
Sana added that the jihadist group had transported troops in from Raqqa, its de facto capital in northern Syria, and the city of Deir Ezzor in the country's east.
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.