The US has said Syria's reactor was similar to a North Korean one
UN nuclear inspectors have arrived in Syria to investigate claims that it was building a nuclear reactor.
The International Atomic Energy Agency's (IAEA) four-person team will spend three days examining the al-Kibar site in the desert in northern Syria.
The site was bombed by Israeli jets in September 2007. The ruins were bulldozed after the attack.
Israel and the US have said the installation was a nuclear plant in the making - a charge denied by Damascus.
Speaking on his departure, the deputy chief of the IAEA, Olli Heinonen - who is leading the team of inspectors - said they would meet their Syrian counterparts on Sunday evening.
After that, he said, they would start "looking for the facts".
Syria has welcomed the inspection but insists that it will be limited to the al-Kibar site.
In April, Washington released pictures purporting to show North Korean experts inside the construction, which it said closely resembled a North Korean reactor at Yongbyon.
Syria has repeatedly denied it has any nuclear weapons programme, or any such agreement with North Korea.
Syrian officials have said the bombed site was an unused military facility under construction, but deny that it had anything to do with a nuclear programme.
IAEA chief Mohamed ElBaradei has criticised both what he saw as a US delay in releasing information on the Syrian site and Israel's bombing of the site before his agency could inspect it.
Ahead of the visit, Mr ElBaradei called on Syria to show "absolute transparency" and to give the inspectors access to all sites they wished to see.
In an interview with Al-Arabiya television, he said: "We have no evidence that Syria has the human resources that would allow it to carry out a large nuclear programme.
"We do not see Syria having nuclear fuel."
CIA briefing video, released in April, comparing images of N Korea and Syria reactors