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STORY: A series of loud explosions rocked a Russian air base near seaside resorts in the annexed Crimean peninsula on Tuesday.

Witnesses said they heard at least 12 blasts, and video obtained by Reuters showed a plume of smoke jetting into the sky. Crimea's health department, along with the Russian governor of Crimea, said at least one civilian had been killed.

But the cause of the blasts remained unclear.

Russia's defense ministry brushed off the idea there had been an attack, and claimed the blasts came from detonations of stored ammunition.

Crimea, a holiday destination for many Russians, was annexed by Moscow from Ukraine in 2014 and used in February as one of the launchpads for its invasion.

On Tuesday - Ukrainian President Volodymyr Zelenskiy did not directly mention the blasts in his daily video address but said it was right that people were focusing on Crimea, and reiterated Kyiv's position that Crimea would have to be returned to Ukraine.

“The presence of Russian occupiers in Crimea is a threat for all of Europe and to the global stability. The Black sea region cannot be a safe place while Crimea is occupied."

If Ukraine were to acknowledge it had attacked territory that Russia sees as its own, Moscow could accuse Kyiv of crossing a red line.

Meanwhile in Washington, U.S. President Joe Biden made a move of his own:

Formally endorsing Finland and Sweden’s accession to NATO, the most significant expansion of the military alliance since the 1990s.

“Our alliance is closer than ever, it is more united than ever, and when Finland and Sweden bring the number of allies to 32, we’ll be stronger than ever.”

The two Nordic countries applied for NATO membership in response to Russia's February invasion of Ukraine.

BIDEN: “Putin thought he could break us apart when this all started, he believed he could break us apart in my view, weaken our resolve. Instead he’s getting exactly what he did not want...”

Moscow has repeatedly warned both countries against joining the alliance.