VIENNA — Around half the single shift of staff who have been working non-stop at the radioactive waste facilities at Chernobyl since Russian forces seized the site last month have been relieved by other Ukrainian staff, the UN nuclear watchdog said on Sunday.

More than three weeks ago Russian forces took control of the waste facilities near the now-defunct power plant that was the site of the world's worst nuclear accident in 1986.

The shift that happened to be working then had been on duty constantly since and unable to rotate out, which the International Atomic Energy Agency had said posed a growing threat to safety as they were exhausted and working under extreme pressure. It has often called for them to be replaced.

"They were there for far too long. I sincerely hope that remaining staff from this shift can also rotate soon," IAEA chief Rafael Grossi was quoted as saying in an IAEA statement issued on Sunday night.

"They had been carrying out their important work tasks under immensely stressful and tiring conditions in the presence of foreign military forces and without proper rest," the IAEA said, adding that Ukraine's nuclear regulator informed it that the rotation had begun on Sunday morning.

"It is a positive – albeit long overdue – development that some staff at (Chernobyl) have now rotated and returned to their families. They deserve our full respect and admiration for having worked in these extremely difficult circumstances," Grossi said. — Agencies