Live updates: follow the latest news on Russia-Ukraine
All members of the one shift of technical staff on duty at Chernobyl's radioactive waste complex since Russian forces seized the site last month have been relieved, the IAEA said on Monday.
For more than three weeks the Ukrainian centre, next to the now-defunct power plant that in 1986 was the site of the world's worst nuclear disaster, were operated by a single shift of staff who happened to be on duty when Russian forces took control on February 24.
All had been unable to leave until Sunday.
For weeks the International Atomic Energy Agency has been saying that the exhausted staff, working under extreme pressure, posed a growing risk to the site's safety and called for them to be rotated out.
"Ukraine's regulatory authority said about half of the outgoing shift of technical staff left the site of the 1986 accident yesterday and the rest followed today, with the exception of 13 staff members who declined to rotate," the IAEA said.
The Ukrainian regulator said most of the guards who have also been there since it was seized remained at the site, the IAEA said.
The agency said last week that there were 211 technical staff and guards there.
The departing technical staff have been replaced by Ukrainian colleagues who are based in the nearby town of Slavutych, the IAEA said.
"The new work shift ... includes two supervisors instead of the usual one to ensure that there is back-up available on the site, the regulator said."
The IAEA said the Ukrainian regulator claimed an agreement had been reached on how to organise staff rotations.