DAMASCUS, Syria — Syrian government forces backed by Russian air strikes advanced swiftly in central Syria on Wednesday, seizing high ground around Palmyra and positioning themselves to recapture the historic town held by the Islamic State group. In Lebanon, the militant Hezbollah group’s television station broadcast footage of the troops, advancing single file through a desert landscape as helicopter gunships provided cover. “God willing, within few hours we will enter and secure the town,” one officer told the Syrian Ikhbariya TV, as a group of soldiers broke into chants in support of President Bashar Assad. In Palmyra, the Islamic State destroyed many of the town’s Roman-era relics, including the 2,000-year-old Temple of Bel and the iconic Arch of Triumph, and also killed dozens of captive Syrian soldiers and dissidents from Islamic State in public slayings at the town’s grand roman theater and other ruins.