Around half of the staff who have been working non-stop at the radioactive waste facilities at the Chernobyl Nuclear Power Plant (NPP) have finally been able to rotate and return to their homes after nearly four weeks, the International Atomic Energy Agency (IAEA) said in a statement on Sunday.

Ukraine's national regulator confirmed that the rotation of on-site personnel had begun on Sunday morning.

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 IAEA warned the shift that happened to be working then had been on duty constantly since and unable to rotate out, which posed a growing threat to safety as they were exhausted and working under extreme pressure.

The UN nuclear watchdog has called for them to be replaced. Director General Grossi, who has expressed deep concern about the well-being of the Ukrainian staff at the site, welcomed the news about the partial rotation of personnel.

"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," Grossi was quoted as saying in the statement.

"It is a positive – albeit long overdue – development that some staff at the Chernobyl NPP 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. "They were there for far too long. I sincerely hope that remaining staff from this shift can also rotate soon."