KYIV, Ukraine (AP) -- A small explosive device carried by a makeshift drone blew up Sunday at the headquarters of Russia's Black Sea Fleet on the Crimean Peninsula, wounding six people and prompting the cancellation of ceremonies there honoring Russia's navy, authorities said.
Meanwhile, one of Ukraine's richest men, a grain merchant, was killed in what Ukrainian authorities said was a carefully targeted Russian missile strike on his home.
There was no immediate claim of responsibility for the drone explosion at the naval headquarters. But the seemingly improvised, small-scale nature of the attack raised the possibility that it was the work of Ukrainian insurgents trying to drive out Russian forces.
The blast took place in the port city of Sevastopol in Crimea, which was seized from Ukraine by Russia in 2014. Observances of Russia's Navy Day holiday were canceled in Sevastopol.
The Black Sea Fleet's press service said the drone appeared to be homemade. It described the explosive device as "low-power." Sevastopol Mayor Mikhail Razvozhaev said six people were wounded.
It was unclear where the drone began its flight. Sevastopol is about 170 kilometers (100 miles) south of the Ukrainian mainland, and Russian forces control much of the mainland area along the Black Sea.
In the wake of the explosion, Crimean authorities raised the terrorism threat level for the region to "yellow," the second-highest tier. However, a Russian lawmaker from Crimea, Olga Kovitidi, told Russian state news agency RIA-Novosti that conclusions about the attack cannot be drawn until an investigation is complete.
Ukraine's navy and an adviser to President Volodymyr Zelenskyy said the reported drone attack underlined the weakness of Russian air defenses.
"Did the occupiers admit the helplessness of their air defense system? Or their helplessness in front of the Crimean partisans?" Oleksiy Arestovich said on Telegram.
If such an attack is possible by Ukraine, he said, "the destruction of the Crimean bridge in such situations no longer sounds unrealistic" -- a reference to the span that Russia built to connect its mainland to Crimea after the annexation.
Elsewhere in Ukraine, the mayor of the major port city of Mykolaiv, Vitaliy Kim, said shelling killed one of Ukraine's wealthiest men, Oleksiy Vadatursky, and his wife, Raisa. Vadatursky headed a grain production and export business.
Another presidential adviser, Mykhailo Podolyak, said Vadatursky was specifically targeted.
It "was not an accident, but a well-thought-out and organized premeditated murder. Vadatursky was one of the largest farmers in the country, a key person in the region and a major employer. That the exact hit of a rocket was not just in a house, but in a specific wing, the bedroom, leaves no doubt about aiming and adjusting the strike," he said.
Vadatursky's agribusiness, Nibulon, includes a fleet of ships for sending grain abroad.
In the Sumy region in Ukraine's north, near the Russian border, shelling killed one person, the regional administration said. And three people died in attacks over the past day in the Donetsk region, which is partly under the control of Russian-backed separatist forces, said regional Gov. Pavlo Kyrylenko.
Podolyak said on Twitter that images of the prison where at least 53 Ukrainian prisoners of war were killed in an explosion on Friday indicated that the blast came from within the building in Olenivka, which is under Russian control.
Russian officials have claimed the building was attacked by Ukraine with the aim of silencing POWs who might be giving information about Ukrainian military operations. Ukraine has blamed Russia for the explosion.
Satellite photos taken before and after show that a small, squarish building in the middle of the prison complex was demolished, its roof in splinters.
Podolyak said those images and the lack of damage to adjacent structures showed that the building was not attacked from the air or by artillery. He contended the evidence was consistent with a thermobaric bomb, a powerful device sometimes called a vacuum bomb, being set off inside.
The International Red Cross asked to immediately visit the prison to make sure the scores of wounded POWs had proper treatment, but said Sunday that its request had yet to be granted. It said that denying the Red Cross access would violate the Geneva Convention on the rights of POWs.