"His recently announced resignation is a concern," Lagarde said in an online news conference when questioned about the resignation of Aivaras Abromavicius yesterday in protest at alleged influence-peddling and state graft.
"If the allegations that he makes in his resignation are correct, then it's obviously an indication that the anti-corruption measures that were committed by the government are not yet working," the IMF managing director said.
The IMF is the main source of rescue financing for Ukraine as the cash-strapped country battles with crises ranging from falling commodity prices to a new trade embargo by Russia.
"There's more progress to be had in this area," Lagarde said.
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"A lot of work needs to be done and it has to be implemented vigorously because the Ukrainian authorities are not only accountable to the Ukrainian people but also to the international community."
Western frustration has been fanned by the fact that Abromavicius is the fourth reform-minded minister to tender his resignation since Ukraine's 2014 revolution broke its ties with Russia and set it on a European course.
President Petro Poroshenko held last-ditch talks with the Lithuanian-born minister Wednesday in a bid to change his mind and reassure him that all his charges would be investigated in full.