US President Donald Trump and his Russian counterpart Vladimir Putin held a lengthy phone call Tuesday, stressing the need to ensure "the stability of a unified Syria," the White House said.

The two presidents emphasized the need "to peacefully resolve the Syrian civil war, end the humanitarian crisis, allow displaced Syrians to return home, and ensure the stability of a unified Syria free of malign intervention and terrorist safe havens," the White House said in a statement.

Unifying Syria is an extremely difficult proposition, with the country currently divided between forces loyal to Kremlin-allied strongman Bashar al-Assad, the US-backed Syrian Democratic Forces, and several jihadist groups and rebel units