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.
The FSB has accused the Ukrainian security services of being behind the attack on the bridge.
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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.
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 forces had aimed to destroy a critically important piece of Russia's civil infrastructure.
Russian forces retaliated on Monday with a wave of missile strikes across the country, including central Kyiv, killing 19 people.
Following more strikes on Tuesday, Ukrainian President Volodymyr Zelensky has urged countries to hit Russia with more sanctions in response to "a new wave of terror".
He called on the West to find new ways to apply political pressure to Russia and support Ukraine.
READ ALSO: Crimea bridge partly reopens after huge explosion - Russia
The calls came after he met the G7 group of nations for emergency virtual talks on Tuesday.
The bloc - which consists of Canada, France, Germany, Italy, Japan, the UK and US - promised to continue providing "financial, humanitarian, military, diplomatic and legal" support to his country "for as long as it takes".
Nato also said it would stand with Ukraine for as long as necessary.
In another development, Polish pipeline operator Pern said it had detected a leak in one pipeline in the Druzhba system that carries oil from Russia to Europe.
The discovery follows leaks in the Nord Stream undersea gas pipelines that transport Russian gas to Europe, which have been widely blamed on sabotage.
Europe is facing a severe energy crisis in the aftermath of Moscow's invasion of Ukraine as it tries to wean itself off Russian gas and oil.
The continent has imposed tough sanctions on Russia in an effort to put economic pressure on the Kremlin.
Pern said that at this point, the causes of the leak were unknown. It was detected in a section of the pipe about 70km from the central Polish city of Plock.
BBC