A drone-deployed explosive detonated at Russia's Black Sea Fleet headquarters, injuring six people, Russian officials announced Sunday.

The attack took place during Russian Navy Day celebrations in Sevastopol, a port city in Crimea that Russia annexed in 2014. The drone appeared to be homemade and the explosive device was relatively low yield. Sevastopol Mayor Mikhail Razvozhaev did not detail the severity of the wounds for the six reported injuries.

It is unclear who is responsible for the attack.

The attack came the same day Russian President Vladimir Putin signed a new Russian naval doctrine in St. Petersburg.

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Navy ships in ocean

Russian Navy ships are docked in the Sevastopol bay on March 4, 2014.  (VIKTOR DRACHEV/AFP via Getty Images) (Photo credit should read VIKTOR DRACHEV/AFP via Getty Images)

Putin with Russia Navy

SAINT PETERSBURG, RUSSIA - JULY,25 (RUSSIA OUT): Russian President Vladimir Putin (C) seen aboard of the ship during the military parade marking the Russia's Navy Day, on July,25,2021, in Saint Petersburg, Russia. The Navy Day military parade held in St.Petersburg, despite the coronavirus (COVID-19) pandemic. (Photo by Mikhail Svetlov/Getty Images) (Photo by Mikhail Svetlov/Getty Images)

Putin stated that Russia intended to cement its status as a global maritime power and named the U.S. as its top global adversary.

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The Russian leader also warned that Russian warships would soon be equipped with hypersonic weapons.

"The key thing here is the capability of the Russian navy," Putin said during a Sunday speech. "It is able to respond with lightning speed to all who decides to infringe on our sovereignty and freedom."

Putin's invasion of Ukraine will finish its fifth full month Sunday as fighting continues across eastern Ukraine.

Ukrainian President Volodymyr Zelensky ordered a mandatory evacuation of the Dontesk region as the area continues to suffer intense fighting. 

"The more people leave [the] Donetsk region now, the fewer people the Russian army will have time to kill," Zelensky said Sunday.

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"Many refuse to leave, but it still needs to be done," he said. "If you have the opportunity, please talk to those who still remain in the combat zones in Donbas. Please convince them that it is necessary to leave."

The Associated Press contributed to this report.