Syrian state media, which has closely covered a three-week push by President Bashar al-Assad to regain the city, aired celebratory footage on Sunday showing government soldiers around the archaeological sites. The Syrian Observatory for Human Rights, a monitoring group, said that there was ongoing fighting in a few districts of the city, as well as at a military prison.

But the majority of the Islamic State contingent in Palmyra had withdrawn or been routed, with hundreds of its fighters killed, the Observatory said, highlighting the extremist group’s broader struggles to retain territory in Syria and Iraq. At the same time, the advance by Mr. Assad’s troops handed him a strategically important military victory that added weight to contention that his government is a crucial bulwark against the jihadists of the Islamic State.

The battle also provided the latest evidence of how significantly Russia’s intervention on behalf of Mr. Assad had transformed his fortunes. The Islamic State fighters had easily taken Palmyra last May from Syrian government troops that had hardly mounted a fight. The recapture of the city on Sunday came after Russia carried out intensive airstrikes in support of the government coalition attacking Palmyra.