The decree did not specify what casualties or consequences, but speculation swirled in Kyiv on Monday that Mr. Bakanov had been ousted for glaring intelligence failures in the first days of the war in the southern city of Kherson, which the Russians captured almost without a fight. Local officials in Kherson switched sides, and explosives were removed from bridges around the city, Mr. Ariev, the opposition member of Parliament said.
In late March, Mr. Zelensky stripped two generals of the security service of their ranks, calling them traitors; one was in charge of the Kherson region and the other fled Ukraine on the eve of the invasion, only to be apprehended months later in Serbia, accused of trying to smuggle cash and emeralds into the country.
One Parliament member, Oleksiy Honcharenko, who is not affiliated with a party, said of Mr. Zelensky’s reference to “grave consequences,’’ “Translation: for the surrender of Kherson.”
The security service, known by its Ukrainian initials S.B.U., is the country’s main domestic security and intelligence authority, Ukraine’s successor to the Soviet-era K.G.B. Its vast size has drawn criticism — by comparison, Britain’s MI5 has just 4,400 employees, according to the Atlantic Council — and it has long faced calls for reform.
Business groups have said that the service shook down companies for bribes and that corrupt agents, compromised and facing possible prosecution, became easy marks for recruitment by Russia.
“Surprise, surprise,” Serhiy Fursa, an analyst with Dragon Capital, a leading Ukrainian investment bank, wrote on Facebook of Mr. Zelensky’s charges of treachery in the service. “What lesson did this war give us? A corrupt man is Putin’s best friend.”
Reporting was contributed by Carlotta Gall from Druzhkivka, Ukraine; Thomas Gibbons-Neff from Dubai; Natalia Yermak from Lviv, Ukraine; Maria Varenikova from Kyiv; and Marc Santora from London.