The cause of his death was not immediately revealed, but back in 2014 he reportedly underwent heart surgery at a Beirut hospital.
Muallem served as Syria’s top diplomat since 2006 while also serving as the country's deputy prime minister since 2012, over almost the entire nearly decade-long foreign-backed militancy in Syria. Back in the '90s he was Syria's ambassador to the United States.
As a long-time ally of Syrian President Bashar Assad, in his public addresses and interviews with foreign media Muallem routinely denounced and exposed foreign interventions and arbitrary violations of Syria’s sovereignty.
Just last month, he criticized “flimsy pretexts” used to hamper the return of Syrian refugees to their homeland, slamming the West for politicizing the purely humanitarian issue to advance their political agendas.