Its FSB security service said five of those held were Russians while the others were Ukrainian and Armenian, BBC reported.

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.

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.

The blast on the Crimea Bridge was a powerful symbolic blow to Russian President Vladimir Putin, who opened the bridge in 2018, four years after Russia's annexation of Crimea.

President Putin called it an "act of terrorism", saying Ukraine's intelligence agents had aimed to destroy a critically important piece of Russia's civil infrastructure, the report said.

But a spokesman for the Ukrainian intelligence services, Andriy Yusov, rubbished the Russian accusations.

"All the activities of the FSB and (Russia's) Investigative Committee are nonsense," he told Ukrainian media.

"They are fake structures which serve the Putin regime, so we're definitely not going to comment on their latest announcements."

Russian forces retaliated on Monday with a wave of missile strikes across the country, including central Kyiv, killing 19 people.

(With UNI inputs)