Delaware - President Donald Trump on Wednesday attended a somber ceremony marking the return of the bodies of two US soldiers and an American civilian who were shot dead in Syria over the weekend.

Trump, Pentagon chief Pete Hegseth, top military officer General Dan Caine, and other officials stood near a gray C-17 transport plane, saluting as three flag-covered cases were carried off the aircraft to a van by white-gloved troops.

US Central Command said a lone gunman from the Islamic State group carried out the attack on Saturday in Palmyra – home to UNESCO-listed ancient ruins and once controlled by the jihadist organization – which also wounded three other service members.

Those killed were Iowa National Guard sergeants William Howard and Edgar Torres Tovar, and Ayad Mansoor Sakat, a civilian from Michigan who worked as an interpreter.

The US personnel were supporting Operation Inherent Resolve, the international effort to combat IS, which seized swaths of Syrian and Iraqi territory in 2014.

The jihadists were ultimately defeated by local ground forces backed by international air strikes and other support, but IS still has a presence in Syria, especially in the country's vast desert.

Trump has long been skeptical of Washington's presence in Syria, ordering the withdrawal of troops during his first term but ultimately leaving American forces in the country.

The Pentagon announced in April that the US would halve the number of US personnel in Syria in the following months, while US envoy for Syria Tom Barrack said in June that Washington would eventually reduce its bases in the country to one.

US forces are currently deployed in Syria's Kurdish-controlled northeast as well as at Al-Tanf near the border with Jordan.