DAMASCUS: A UN investigator expressed hope Thursday (Jan 9) of a "good relationship" with Syria's new rulers as he wrapped up a first visit by his inquiry team, which was barred by ousted president Bashar al-Assad 13 years ago.

"It is a huge surprise to be in Damascus," said Hanny Megally of the UN Commission of Inquiry on Syria (COI), expressing optimism that "we have an interlocutor we can work with".

"We hope to be able to have a good relationship with the authorities now in place," he told AFP.

The COI has been gathering evidence of crimes committed in Syria since the early days of the civil war triggered by the Assad government's crackdown on pro-democracy protests in 2011.

Assad never granted the investigators permission to enter Syria, but the new authorities did so "immediately", Megally said.

Previously, the COI had been conducting its investigation remotely. Based on interviews with thousands of people and large archives of documents, it has drawn up a list of 4,000 suspected perpetrators of serious crimes.

During his five-day visit, Megally said he had toured several notorious detention centres and mass grave sites around Damascus and met with justice and foreign ministry officials.

He expressed hope that the new authorities would "recognise the work we've been doing is actually of good use for them, both in dealing with the accountability and justice going forward".

"We want to be able to now go to the places we were documenting without access to the country to reconfirm, in essence, the information we've been gathering" and fill the gaps, he said.

"I think we've covered practically every war crime that people could think of, but it wouldn't surprise me if something else emerges - but hopefully not."

The COI wants "to help ensure that the past is not repeated", said Megally, whose remit is to investigate all parties to the Syrian civil war, including the former rebels who are now in charge.

"There's no party to the conflict in Syria that can say: 'We observed human rights law or we observed international humanitarian law'," Megally said.

"But we're now in a new phase ... it would be good to be able to say lessons have been learnt ... where we don't repeat the mistakes of the past."