The United States on Monday welcomed the victory by Syrian leader Bashar al-Assad's Russian-backed forces over the Islamic State in the historic city of Palmyra, despite concerns over his brutal record.

"We do think that it's a good thing that Daesh no longer controls it," State Department spokesman John Kirby said, using his preferred term for the Islamic State group.

"That said, we're also mindful, of course, that the best hope for Syria and the Syrian people is not an expansion of Bashar al-Assad's ability to tyrannize the Syrian people."