Syrian government forces have regained control of Palmyra, the historic Syrian city that was overrun by the Islamic State in May of last year.

Syrian state media reported Sunday that government troops, backed by Shia militias loyal to Syrian President Bashar al-Assad on the ground and Syrian and Russian strikes from above, drove ISIS militants out of the city overnight. Soldiers found and dismantled hundreds of bombs and other explosive devices planted by the Sunni extremist group throughout the city, which is located in the eastern countryside of the Homs province. State media estimates about 450 ISIS militants were killed in the operation to recapture Palmyra.

The Syrian Observatory for Human Rights, a British-organization that documents activity in Syria with the help of activists in the country, confirmed Sunday that government troops had pushed ISIS fighters out of the city.

Russia’s ministry of defense said Saturday Russian warplanes carried out 40 sorties and killed more than 100 militants in the operation.

Syrian government forces had been on the offensive for nearly three weeks in an attempt to gain control of Palmyra, which was seized by ISIS in May. Last week, troops recaptured Palmyra Castle, an ancient citadel.

Palmyra, a UNESCO World Heritage Site, is home to tens of thousands of people and a trove of ancient ruins. Last summer, ISIS fighters began pillaging the city’s antiquities, saying the ruins violate the Islamic injunction against idol worship. They destroyed the Temple of Baalshamin and the Arch of Triumph, structures that were nearly 2,000 years old. UNESCO, the United Nations that documents the world’s historic and cultural sites, called the destruction a war crime.

In August, ISIS fighters beheaded Khaled al-Asaad, an 82-year-old antiquities scholar who reportedly refused to reveal the location of valuable artifacts in Palmyra that were hidden before militants swarmed the city.

Stills from Russian television provided to Reuters on Sunday showed aerial views of the ruins of Palmyra. Maamoun Abdulkarim, the director of the museums and antiquities department in Damascus, said in a message to ISIS that “we will rebuild what you have destroyed,” the Associated Press reported.

Assad on Sunday described the Palmyra operation as a “significant achievement” offering “new evidence of the effectiveness of the strategy espoused by the Syrian army and its allies in the war against terrorism,” according to state media.

A member of the anti-Assad opposition coalition rebuked that assessment. “The government wants through this operation to win the favor of Western nations by fighting against terrorism, while obscuring its responsibility as providing the reasons for the spread of terror,” Khaled Nasser told the AP.

The Syrian government and opposition groups are currently engaged in United Nations-mediated peace talks in Geneva aimed at resolving the country’s civil war, which has left more than 250,000 people dead in over five years. The various factions agreed to a temporary cease-fire late last month that has mostly held despite some outbreaks of violence. Both the peace talks and that accord do not include ISIS, which lost ground in Syria after Russia began launching airstrikes on Assad’s behalf last fall, turning the conflict in the Syrian president’s favor. Russian President Vladimir Putin earlier this month withdrew a major part of Russian forces backing Assad’s troops in Syria, but has continued bombing targets there.