North Korea is making "important" progress on reactivating facilities at its moth-balled Yongbyon nuclear reactor, a U.S. think-tank says.

Start-up could be one to two months away, it said, but there was uncertainty over the availability of fuel rods to power the reactor.

North Korea announced in April that it planned to reactivate the reactor, amid high tension on the peninsula.

The reactor was closed in July 2007 as part of a disarmament-for-aid deal.

The cooling tower at the facility was later destroyed, but then the disarmament deal stalled.

North Korea's decision to restart followed its third nuclear test on 12 February, which led to expanded UN sanctions, BBC News reported.