POKROVSK, Ukraine— A Russian airstrike on residential areas killed at least 21 people early Friday near the Ukrainian port of Odesa, authorities reported, a day after the withdrawal of Moscow’s forces from an island in the Black Sea had seemed to ease the threat to the city.

Video of the attack before daybreak showed the charred ruins of buildings in the small town of Serhiivka, about 50 kilometers (31 miles) from Odesa. The Ukrainian president’s office said warplanes fired three missiles that struck an apartment building and a campsite.

Ukrainian authorities interpreted the attack as payback for the withdrawal of Russian troops from Snake Island a day earlier, though Moscow portrayed their departure as a “goodwill gesture” to help unblock exports of grain.

Russian forces took control of the island in the opening days of the war in the apparent hope of using it as a staging ground for an assault on Odesa, Ukraine’s biggest port and the headquarters of its navy.

“The occupiers cannot win on the battlefield, so they resort to vile killing of civilians,” said Ivan Bakanov, head of Ukraine’s security service, the SBU. “After the enemy was dislodged from Snake Island, he decided to respond with the cynical shelling of civilian targets.” Ukraine’s military reported late Friday on social media that two Russian Su-30 warplanes bombed Snake Island with phosphorus bombs. Black-and-white aerial video showed two blasts hitting the island. The warplanes reportedly struck from the east, from Belbek, on the Russian-occupied Crimean Peninsula. The Russian military did not immediately comment.

Large numbers of civilians were killed in Russian bombardments earlier in the war, including at a hospital, a theater used as a shelter, and a train station. Until this week, mass casualties involving residents appeared to become less frequent as Moscow concentrated on capturing eastern Ukraine’s Donbas region. (AP)