North Korea appears to be still expanding operations at its main nuclear site, the UN atomic watchdog indicated on Thursday.

The statement by the head of the International Atomic Energy Agency (IAEA) is a further sign that North Korea is pressing ahead with its atomic activities despite pressure from the United States for it to scrap its nuclear weapons program.

At a summit in June, U.S. President Donald Trump and North Korean leader Kim Jong-un pledged to work toward the complete denuclearization of the Korean Peninsula.

Since then, North Korea appears to have begun dismantling a pair of missile testing and launching facilities, according to a report published by North Korea analysis website 38 North in July. But UN experts have warned the country has not moved toward the sort of full, irreversible denuclearization program Washington has called for.

At Yongbyon, North Korea's main nuclear facility that is widely believed to have provided fissile material for its bombs, components appear to have been brought into a light-water reactor being built there, IAEA chief Yukiya Amano said.

"At the light-water reactor, the agency also observed activities consistent with the fabrication of reactor components and the possible transfer of these components into the reactor building," Amano said in a statement to the IAEA's board of governors, reporting on the period since August.

No access for monitors since 2009

The IAEA has not had access to North Korea since 2009 and mainly monitors activities there through satellite imagery, meaning it cannot confirm its observations.

North Korea also appeared to have continued work at the nearby Kuryong River that it dammed last year to increase the supply of water available for cooling the planned reactor or the existing experimental one, which has an output capacity of five megawatts.

"Further activities were observed near the Kuryong River. These may be related to changes to the cooling infrastructure for the five-megawatt reactor and the light-water reactor," Amano said, adding that the experimental reactor was likely shut down while some of those unspecified activities were carried out.

Apart from an annual report on developments in North Korea, the IAEA does not normally comment on what it sees there, suggesting that Amano's comments were aimed at illustrating the continued activity at Yongbyon.