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 on Sunday in a courtyard at the naval headquarters in the city of Sevastopol. 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.

A Russian lawmaker from Crimea, Olga Kovitidi, told Russian state news agency RIA-Novosti that the drone was launched from Sevastopol itself. She said the incident was being treated as a terrorist act, the news agency said.

Crimean authorities raised the terrorism threat level for the region to "yellow," the second-highest tier.

Sevastopol, which was seized along with the rest of Crimea from Ukraine by Russia in 2014, is about 170 kilometres (100 miles) south of the Ukrainian mainland. Russian forces control much of the mainland along the Black Sea.

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. Observances of Russia's Navy Day holiday were cancelled in the city.

Ukraine's navy and an adviser to President Volodymyr Zelenskyy said the reported drone attack underlined the weakness of Russian air defences.

"Did the occupiers admit the helplessness of their air defence 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 organised 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, saying it was done to cover up the torture and execution of prisoners.

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.

Meanwhile, Zelenskyy said that the war has significantly reduced the size of Ukraine's grain harvest compared with past years, but that Ukraine is working on ways to export what it has to avoid a global food crisis.

"The Ukrainian harvest this year is in danger of being half as large," he said on Twitter.

Russia and Ukraine recently reached an agreement that would allow the release of millions of tons of gra