ISIS second-in-command is believed to have been killed in a US air strike.

Highlights Abd ar-Rahman Mustafa al-Qaduli killed in US air strike in Syria: reports

The commander used several aliases

ISIS has blown up UNESCO-listed temples dated back thousands of years

The ISIS group suffered a double setback in Syria on Friday as army troops recaptured the ancient citadel in Palmyra and the Pentagon said the jihadists' second-in-command was killed in a US raid.The fresh gains in Palmyra by Syrian troops backed by allied militia and Russian warplanes came nearly a year after IS overran the UNESCO world heritage site, sparking global concern.The regime advance came after US Secretary of State John Kerry and Russia's President Vladimir Putin agreed to intensify the drive for a political settlement in Syria."Our armed forces, in coordination with the popular defence forces, have taken control of the ancient Palmyra citadel after inflicting many losses in the ranks of the terrorist group Daesh," Syrian state television said, using another name for IS.The army has also cut off the main Palmyra-Deir Ezzor highway leading to the Iraqi border, the report said citing a military source.The jihadist group had taken over the citadel on May 23 last year and raised its notorious black and white flag over it.The group has since blown up UNESCO-listed temples and looted relics that dated back thousands of years, and murdered the former antiquities chief in Palmyra, Khaled al-Assaad.Built in the 13th century, the citadel is Palmyra's main Islamic-era monument.Earlier, Syria's antiquities chief Maamoun Abdelkarim told AFP the army had also made gains elsewhere in the city."In the southwest, the army has liberated the district of hotels and restaurants as well as the Valley of the Tombs," he said.The Syrian Observatory for Human Rights, a Britain-based monitor, reported that Russian and Syrian warplanes were continuing air strikes in the area."The army is now 600 metres from the Temple of Bel, but it is advancing slowly because of mines and above all to protect the city, which is an ancient treasure," Abdelkarim said.IS claimed in September to have destroyed the Temple of Bel, which UNESCO had described as one of the best preserved and most important first century religious edifices in the Middle East.Palmyra's full recapture would be a major strategic and symbolic victory for President Bashar al-Assad, since whoever holds it also controls the vast desert extending from central Syria to the Iraqi border.The advance came as Pentagon chief Ashton Carter announced the killing of Abd ar-Rahman Mustafa al-Qaduli, described as IS's second-in-command, in a US air raid in Syria."The removal of this ISIL leader will hamper the ability for them to conduct operations inside and outside of Iraq and Syria," Carter told reporters, using an alternative acronym for the group.IS also faces mounting pressure in neighbouring Iraq where the army said on Thursday it had launched a long-awaited offensive to retake the second city of Mosul, a key IS hub since a lightning assault in June 2014.Experts have warned that any battle to retake Mosul would be difficult, given the significant number of jihadists and civilians in the city and the time IS has had to prepare defences.The fighting came as the latest round of peace talks aimed at ending Syria's five-year war, which has left more than 270,000 people dead, came to a close.Kerry and Putin, who back different sides in Syria's war, agreed at a rare meeting in Moscow to push for a political settlement, but the future of Assad remains a sticking point.Unless Assad agrees to step down, there are concerns that the Syrian opposition could drop out of peace negotiations, which UN envoy Staffan de Mistura said he was aiming to restart on April 9.'Symbol of cultural cleansing'The focus of Syria's war appears to have shifted to the battle against IS in Palmyra, nearly a month after a ceasefire between the army and non-jihadist rebels brokered by the United States and Russia came into force.Global concern over the jihadist threat was further heightened this week by a deadly attack in Brussels that was claimed by IS.In IS's de facto capital Raqa in northern Syria, "jihadists have been celebrating the Brussels attack all week," Observatory director Rami Abdel Rahman told AFP.The group's top religious figure in the city, Abu Ali al-Sharii, meanwhile led the Friday prayer with a pledge to commit more violence."We vow new operations by jihadists in the West," he said, according to the Observatory.UNESCO chief Irinia Bokova welcomed the push to recapture Palmyra."For one year, Palmyra has been a symbol of the cultural cleansing plaguing the Middle East," Bokova said.