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A senior US official met with Syrian regime's national security chief in Damascus earlier this week, in the highest ranking visit to Syria by a Washington personnel since the start of the war in 2011.

The unnamed US official travelled to Damascus via Lebanon, and held talks with Syria's notorious national security chief, Ali Mamlouk, according to the Lebanese newspaper al-Akhbar.

The two discussed security matters and the fate of missing Americans in Syria, including operatives of the Central Intelligence Agency (CIA).

But the source who spoke to Reuters said "it is an important step, but Damascus does not have confidence in the American position", the regional official said.

Since the conflict broke out in 2011, the United States has supported some opposition groups fighting Bashar al-Assad, providing some anti-regime rebels with weapons via a CIA-run military aid programme.

President Donald Trump ordered the programme to shut down earlier this year.

But despite the latest meeting, US Secretary of State Rex Tillerson reiterated his country's position that Assad should leave power.

He said "reign of the Assad family is coming to an end" and "the only issue is how that should be brought about".

Seven rounds of talks have achieved only incremental progress toward a political deal, with negotiations deadlocked over the fate of Assad.

The opposition is demanding that any settlement provide for a transition of power to end Assad's rule. As government forces make gains on the battleground, there is little likelihood of a breakthrough on that issue.

The US' top diplomat said that an exit of Assad should be done through the Geneva process led by de Mistura, but that such a departure was not a "prerequisite" for that process to start.

Assad has recaptured swathes of Syrian territory lost since the conflict began with military backing from Russia and Iran over the last two years.

US forces have also been fighting in Syria as part of the coalition against Islamic State, helping Kurdish-led militias capture Raqqa and other parts of northern and eastern Syria from the militants. 

During the meeting, Mamlouk allegedly said US forces "are on Syrian land and this is considered occupation", according to the regional official.

But the US official responded stating "our presence is advisory and we are fighting Daesh", the regional official added, using an Arabic acronym for the Islamic State militant group.