Putin informs Trump about talks with Assad: Kremlin

Putin informed Trump of "the main results" of his meeting with Assad, held in the Black Sea resort of Sochi on Monday, "at which the Syrian leader confirmed his commitment to the political process, (and) conducting constitutional reform and presidential and parliamentary elections," the Kremlin said.

During the phone call with Trump, Putin stressed the "need to keep Syria's sovereignty, independence and territorial integrity intact," the Kremlin said.

A political settlement in Syria should be based on principles to be worked out during an all-inclusive "internal negotiating process".

The White House called for the need to ensure "the stability of a unified Syria," a statement said.

The aim is "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," it added.

But there was no mention of Assad's future.

The Syrian president's fate remains a huge stumbling block, preventing global players from reaching a peace settlement over the six-year war.

The Syrian conflict began when the Baath regime, in power since 1963 and led by Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.

Putin's talks with Assad were part of Moscow's effort to reboot the Syrian peace process, after military successes on the ground.

Putin's flurry of diplomacy also included phone talks with Saudi Arabia's King Salman and the leaders of Egypt and Israel, ahead of a trilateral summit with the presidents of Turkey and Iran on Wednesday.