Ukraine told the International Atomic Energy Agency that around half of the staff were "finally" able to return to their homes on Sunday after working at the Russian-controlled site for nearly four weeks, IAEA director general Rafael Grossi said.

Those who left were replaced by other Ukrainian staff, Grossi said in a statement late Sunday.

"It is a positive -- albeit long overdue -- development that some staff at the Chernobyl NPP have now rotated and returned to their families," Grossi said.

"They deserve our full respect and admiration for having worked in these extremely difficult circumstances. They were there for far too long. I sincerely hope that remaining staff from this shift can also rotate soon."

On February 24, the day Russia invaded Ukraine, Moscow's troops seized the Chernobyl compound, the site of the 1986 core meltdown that sparked the worst nuclear reactor catastrophe in history.

Around 100 technicians have been working under armed guard to maintain the site since then.

Grossi, who had expressed deep concern about the well-being of the Ukrainian staff at the site, "welcomed the news about the partial rotation of personnel," the IAEA said.

"Before today's rotation, the same work shift had been on-site since the day before the Russian forces entered the area," it continued.

It is unclear why Russian soldiers seized Chernobyl, where the destroyed reactor is kept under close supervision within a concrete and lead sarcophagus, and the three other reactors are being decommissioned.

In 2017, the site was one of several Ukrainian targets hit by a massive cyberattack thought to have originated in Russia, which briefly took its radiation monitoring system off-line.