In the earlier post, the authorities had said eight people were killed.
The bus, which was attacked near the city of Bilopillya while travelling towards Sumy, was “targeted by the Russians”, the military administration said.
The blast came hours after Russia and Ukraine concluded their first direct talks in almost three years in Istanbul, Turkey, on Friday, but without a significant breakthrough.
The two sides agreed a large-scale prisoner exchange but no ceasefire, saying instead that they would commit to trading ideas on a possible truce.
After the talks, Kyiv said it was seeking an “unconditional ceasefire” to pause the conflict, which has destroyed large swathes of Ukraine and displaced millions of people.
But Moscow has consistently rebuffed those calls and the only concrete agreement appeared to be a deal to exchange 1000 prisoners each.
Ukraine’s top negotiator, Defence Minister Rustem Umerov, said the “next step” would be a meeting between Russian President Vladimir Putin and his Ukrainian counterpart Volodymyr Zelenskyy.
Moscow and Washington have also talked up the need for a meeting on the conflict between Putin and US President Donald Trump.
– Agence France-Presse