A Russian missile attack on Odesa killed at least 20 people and injured 73 others, Ukrainian authorities said on Friday, the latest in a series of deadly air assaults on the southern Ukrainian port city.

Ukraine’s state emergency services said a first missile hit several houses late in the morning, prompting rescuers to rush to the scene. A second missile then landed on the same site, causing many fatalities, including at least one paramedic and a rescue worker. The reports could not be independently verified.

Oleh Kiper, the governor of the Odesa region, posted photos on social media showing rescue workers evacuating one of their colleagues on a stretcher and trying to put out a fire near a destroyed building. A photo released by the Odesa City Council showed what appeared to be a rescuer lying on the grass, his lifeless body covered by a foil blanket.

Ukrainian authorities said the attacks destroyed a three-story building, damaged 10 houses and a gas pipeline, and started a fire that spread to an area of about 1,300 square feet.

It was the third deadly assault on Odesa in two weeks, with a total of at least 38 people killed. It came as Russians began voting in a presidential election that President Vladimir V. Putin was all but certain to win, and while his country’s war in Ukraine had entered its third year and showed no sign of abating.

On the same day, Russian authorities said Ukrainian shelling of the western city of Belgorod, close to Ukraine, had killed one civilian and wounded two others. Their claims could not be independently verified.

The Belgorod region has been the scene of cross-border ground attacks by Ukraine-backed Russian groups this week, apparently aimed at disrupting Mr. Putin’s re-election campaign messaging that the war had turned in Moscow’s favor. Russian forces have had the advantage on the battlefield in recent months, attacking simultaneously in several places all along the front line.

The assault on Odesa on Friday appeared to be what military officials call a “double-tap attack,” hitting the same target twice with a time gap between the two strikes in order to kill emergency workers or firefighters responding to the first strike. Russian forces have used this tactic before in Ukraine and Syria.

Videos posted online by Ukrainian reporters in Odesa showed bodies lying in pools of blood on the pavement, covered with a foil blanket, a simple sheet or a body bag. Andrii Vahapov, a member of the Odesa City Council, said those killed included Odesa’s former deputy mayor, Serhii Tetiukhin, and Oleksandr Hostishchev, the commander of a police special forces unit.

Natalia Humeniuk, a spokeswoman for the Ukrainian Army in the south, said the attack had involved ballistic missiles fired from Crimea, the Russian-occupied Ukrainian peninsula.

Odesa, a lifeline for the Ukrainian economy, is home to a vast port infrastructure vital to Ukraine’s Black Sea exports. Many vessels depart from the port to ship grain and other agricultural products across the Black Sea.

The city, which Mr. Putin has long claimed belongs to Russia, was relatively untouched by the fighting at the start of the war. But over the past six months, Russia has targeted grain silos and naval infrastructure there with drones and missiles in an attempt to undermine Ukraine’s relatively successful Black Sea export operations.

The attacks have also caused civilian casualties, either from direct targeting or falling debris. Earlier this month, a drone hit a residential building, killing 12 civilians. It took rescue workers several days to pull the bodies from the rubble, including those of babies and children.

And last week, a missile hit the city while President Volodymyr Zelensky and Prime Minister Kyriakos Mitsotakis of Greece were visiting, killing five people. Mr. Zelensky later told an Italian television station that the missile hit less than half a mile from the place where he was standing with Mr. Mitsotakis.

“I don’t know who this hit was aimed at,” Mr. Zelensky said, adding that it was “incredible” that Russia could target the city while state leaders were visiting. The attack drew widespread condemnation from foreign leaders.

Reacting to Friday’s strikes on Odesa, Mr. Zelensky said in a video statement that the double-tap tactic made it “a very vile attack.” He added that rescuers were continuing to search for people under the rubble.