International corruption experts say they are alarmed by the situation involving a detained agent from the National Anti-corruption Bureau of Ukraine (NABU) and believe the action is "a step back" for Ukraine in its efforts to fight corruption.

"As regards yesterday's events, we are concerned that it may harm the work done over the course of the last year regarding fighting corruption in Ukraine," Spanish prosecutor Carlos Castresana said during a briefing in Kyiv on Thursday.

In turn, chairman of the working group on bribery and international commercial operations at the Organization for Economic Cooperation and Development (OECD) Drago Kos said, "They [Ukraine's SBU Security Service and Prosecutor General's Office (PGO)] arrested a NABU detective and ruined the investigation… They damaged the credibility of NABU to conduct anti-corruption activities."

According to Kos, the incident may have "serious consequences" for NABU's future work. "This may be the beginning of more serious infringements on NABU, which had just started to produce results."

Ukraine's State Migration Service (SMS) earlier reported the arrest of a NABU agent during an attempt to give a bribe SMS deputy head Dina Pimakhova.

"For several months the NABU agent, the son of an ex-Verkhovna Rada deputy from the Party of Regions Yuriy Boyarsky, fulfilling the illegal directives of some NABU officials, attempted to provoke Pimakhova, offering her large sums of money in return for illegal decisions on legalizing citizens of Vietnam and Iran in Ukraine who didn't even exist," SMS' press service said.

PGO chief Yuriy Lutsenko on Thursday said the NABU agent would be served a notice of suspicion, adding that no one would be arrested. Prosecutors are expected to interrogate a minimum of two high-ranking NABU officers on Monday.