Russian forces launched waves of drones at the Odesa region of southern Ukraine in an hourslong overnight assault, officials said on Sunday, the latest bombardment to target port infrastructure since Moscow pulled out of a deal allowing safe passage for Ukrainian grain through the Black Sea.

Serhii Bratchuk, a spokesman for the Odesa military administration, said that port infrastructure on the Danube River had been hit and that two port employees were injured in the attack, which lasted more than three hours and involved more than two dozen drones. Ukraine’s Air Force said it shot down 22 out of 25 attack drones and the State Emergency Service posted photos of firefighters in the Odesa region trying to extinguish a blaze.

Mr. Bratchuk did not specify where exactly the strikes landed, but local Ukrainian media reported explosions in the port city of Reni on the Danube, just across the water from Romania. Russia’s ministry of defense claimed that its drones had struck fuel storage facilities there; the claim could not be independently verified.

Andriy Yermak, the head of the Ukrainian president’s office, condemned the overnight attack. In a statement on the Telegram messaging app, he accused Russian forces of targeting port infrastructure “in the hope they will be able to provoke a food crisis and hunger around the world.”

Ukraine’s main Danube ports represent a potentially perilous tripwire, because they lie so close to Romania, a member of NATO, and therefore to territory covered by the alliance’s commitment to collective security. On Sunday, Romania’s defense ministry said it had been monitoring the overnight drone attacks in real time and denounced what it called “unjustified” assaults on infrastructure in Ukraine.

For years, Ukraine’s Danube ports played a secondary role, with the primary conduit for the country’s grain exports being Black Sea ports such as the one in the city of Odesa. But that changed when Russia pulled out of the Black Sea grain agreement in July, threatening all ships moving to and from Ukraine.

The Danube delta became an immediate alternative waterway for grain ships. But then Russia began attacking the smaller ports on the river as well, bombing Ukrainian grain-loading facilities there. In mid-August, granaries and warehouses in Reni and Izmail, another port on the river, were damaged as a result of Russian attacks.

In an attempt to get exports moving again, Kyiv established a temporary corridor hugging the western Black Sea coast from Ukraine to Turkey, to allow passage for civilian ships that have been stuck in Ukrainian ports since before Russia’s full-scale invasion.

A handful of vessels have used the corridor in recent weeks, and President Volodymyr Zelensky of Ukraine said on Saturday that two more ships had successfully navigated passage. He later hailed Odesa as “a port on which the lives of various nations depends” in his overnight address, just hours before the latest strikes.

The attacks in the Odesa region came amid international efforts to revive the grain deal. Russia has been touting what it casts an alternative to the agreement, which was brokered by the United Nations and Turkey and helped stabilize food prices across the world but which Moscow complained was carried out unfairly.

Precise details of the Russian proposal remain scant, but President Vladimir V. Putin is scheduled to discuss the matter with Turkey’s leader, Recep Tayyip Erdogan, on Monday when the two leaders hold bilateral talks in the Black Sea resort of Sochi.

Here’s what else is happening in the war:

• Ukrainian officials said a Russian strike had hit a residential building in the eastern town of Vuhledar on Saturday, killing a man and his wife. The prosecutor general’s office said on Telegram that the couple’s 19-year-old daughter and another resident of the town were injured.