Russia says it has detained eight people in connection with Saturday’s explosion on a key bridge linking Russia to Crimea.
Its FSB security service said five of those held were Russians, while the others were Ukrainian and Armenian.
It says Kyiv was behind the attack but a Ukrainian official described Russia’s investigation as “nonsense”.
The news came as explosions were reported in the Ukrainian cities of Kherson, Zaporizhzhia and Nikopol.
The BBC’s Hugo Bachega in Kyiv said five explosions had been heard in Kherson, one of the largest cities under Russian occupation, while there were unconfirmed reports that the air defence system in the city had been activated.
He said it was not clear what had triggered the explosions.
Ukraine’s military said its troops were continuing their advance in the region, capturing another five settlements.