(Adds Slovak candidate)
By Francois Murphy
VIENNA, Sept 4 (Reuters) - The head of the global nuclear test-ban agency is running to become the next director general of the U.N. atomic watchdog, he told staff on Wednesday, bringing the number of candidates to four from three continents.
International Atomic Energy Agency Director General Yukiya Amano died in July as he was preparing to step down early because of an unspecified illness that had visibly weakened him over the previous year.
His right-hand man, Cornel Feruta of Romania, is now acting director general and running to take over the position fully against Rafael Grossi, a veteran of nuclear diplomacy, currently Argentina's ambassador to the Vienna-based IAEA.
Slovakia's government has also approved a run by the head of its Nuclear Regulatory Authority, Marta Ziakova, a former chairwoman of the IAEA's two top decision-making bodies, the Board of Governors and the General Conference.
After weeks of speculation about a possible bid, Lassina Zerbo of Burkina Faso, who heads the Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO), told staff in a meeting at CTBTO headquarters in Vienna that he is running with the support of "his country and his region", a person in the meeting said.
Romania's Feruta has told member states that he represents "continuity, credibility, impartiality and transparency", and Grossi has said he would ensure "transparency and a fluid and direct dialogue with all" https://www.cancilleria.gob.ar/en/announcements/news/letter-argentine-candidate-iaea-dg - an apparent reference to what some diplomats saw as excessive secrecy under Amano.
Zerbo and Ziakova have yet to circulate platforms as widely.
Diplomats who follow the IAEA do not expect major shifts in how the agency handles its most high-profile issues, such as inspections in Iran and its sought-after return to North Korea.
Zerbo drew the ire of IAEA heavyweight Russia last month by saying where and when Russian CTBTO monitoring stations that scan the air for radioactive particles went offline after a mysterious explosion during a rocket engine test there.
Zerbo also posted a simulation https://twitter.com/SinaZerbo/status/1163094836569882625 of the potential path taken by a plume of gas from the explosion, which made it clear that several stations' communications shut down as the plume was believed to have been approaching.
That fuelled suspicions that Russia tampered with the stations in an effort to hide information about the nature of the blast, which caused a spike in radiation in a nearby city. U.S.-based nuclear experts have said they suspect Russia was testing a nuclear-powered cruise missile.
Russia told the CTBTO the explosion was none of its business and handing any radiation data to it was voluntary.
The deadline to nominate candidates is Thursday. The IAEA's 35-nation Board of Governors aims to choose Amano's successor next month. (Additional reporting by Tatiana Jancarikova in Bratislava Editing by Mark Heinrich)