North Korea is making "important" progress on reactivating facilities at its moth-balled Yongbyon nuclear reactor, a US 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.
Pyongyang vowed to restart the reactor, which makes weapons-grade plutonium, in April amid severe regional tensions.
The reactor was shut down 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.
The information came from the 38 North website, which is part of the US-Korea Institute at Johns Hopkins University in the US.
Its report said that recent satellite imagery showed that North Korea had "essentially finished repairing the cooling system necessary to restart and operate the reactor".
The cooling tower that was destroyed had not been repaired, but instead a secondary cooling system had been employed. Work was also ongoing at a spent fuel facility, it said.
Piles of construction materials were visible at the site and what could be a new drainage ditch for water from the reactor building was being dug, it said.
The reactor "may be one to two months from start-up. However, the availability of fresh fuel rods to power the reactor - a key factor that will determine when the North will restart the facility - remains unclear," it said.
Once operational, the reactor could produce "approximately six kilograms of plutonium per year that can be used for manufacturing nuclear weapons", it added.
North Korea has conducted three nuclear tests since 2006. Analysts believe the first two tests used plutonium as the fissile material, but it is not known whether the third used plutonium or uranium.
While North Korea has depleted its stocks of "reactor-grade" plutonium needed to make the weapons-grade variety, it has plentiful reserves of uranium ore. It also has a uranium enrichment facility at Yongbyon which a US scientist said could be converted to produce highly enriched uranium bomb fuel.
After UN sanctions were expanded following the most recent nuclear test, in February, North Korea issued multiple threats against US and regional interests, vowed to reactivate Yongbyon and cut both official communications and key business ties with South Korea.
Operations at the jointly-run inter-Korean Kaesong industrial zone remain suspended - the first such stoppage since the project began.
But the threats have diminished in recent weeks and last month, North Korea sent a top envoy to Beijing - its first such move since its nuclear test.
Later this week, the US and Chinese presidents meet in California for their first summit, with North Korea likely to be high on the agenda.