VIENNA (Reuters) - The purpose of an annex being built next to a suspected uranium enrichment plant at North Korea's Yongbyon nuclear site remains unclear, U.N. nuclear watchdog chief Rafael Grossi told a meeting of his agency's Board of Governors on Monday.

       "We continue to observe construction activities at the Yongbyon site, including construction of an annex to the reported Centrifuge Enrichment Facility, the purpose of which has yet to be determined," he said in a brief update on North Korea, which his agency has no access to, in a longer statement.

       (Reporting by Francois Murphy; Editing by Toby Chopra)