Syria has offered to cooperate with a UN investigation into evidence that it tried to build a reactor that could have been used to make a nuclear weapon, it has been reported.

For the past three years, the country has prevented inspectors from the International Atomic Energy Agency gaining access to the site at al-Kibar.

The offer of cooperation comes as the government of President Bashar al-Assad is trying to suppress demonstrations inspired by the Arab spring democratic movement.

A damning IAEA report last week on the suspect building, which was bombed by Israel in 2007, said the IAEA "assesses that it is very likely that the building destroyed ... was a nuclear reactor". This is a far stronger finding than previous reports, which simply registered unanswered questions about the site.

Last week's report also noted Syria's lack of cooperation with the IAEA after an initial inspection found traces of man-made uranium, and gave details of an apparent attempt to conceal, dismantle and bury the remains of the building. The report prompted US-led calls for Syria to be referred to the UN security council.

According to documents seen by the Associated Press, the IAEA chief, Yukiya Amano, reported on Friday that top Syrian nuclear officials had told him the previous day: "We are ready to fully cooperate with the agency."

Despite the offer, which some diplomats regard as a delaying tactic, the US will seek Syria's referral to the security council when the IAEA's governing board meets on 6 June.

A US letter sent to IAEA board members on Friday, and obtained by AP with an attached copy of a draft resolution, said: "Such cooperation would indeed be welcome but would not have any bearing on the finding of non-compliance" by Syria of its obligations under the nuclear non-proliferation treaty.

The outcome of the IAEA's vote on the US resolution is unclear, as the Syrian offer may persuade some members that a referral is not warranted.

• This article was amended on 30 May 2011. A sentence in the original quoted an IAEA report as saying that the agency "assesses that the building destroyed ... was a nuclear reactor". This has been corrected.