Syria's Bashar al-Assad said in an interview aired on Tuesday that Donald Trump will be a "natural ally" if the US president-elect fulfils his pledge to fight "terrorists".

In his first reaction to Trump's election victory last week, the Syrian president struck a note of caution and said he was unsure the American billionaire would be able to keep his word and step up the fight against jihadists.

"We cannot tell anything about what he's going to do, but if... he is going to fight the terrorists, of course we are going to be ally, natural ally in that regard with the Russian, with the Iranian, with many other countries," he told Portugal's RTP state television.

Asked about Trump's campaign comments suggesting the United States should focus more on fighting the Islamic State (Isil) group, Assad said he would welcome such a move but was cautious.

"I would say this is promising, but can he deliver?" said Assad, who was speaking in English.

"Can he go in that regard? What about the countervailing forces within the administration, the mainstream media that were against him? How can he deal with it?" he said.