Ukraine's air force on Wednesday said a large group of Russian army drones entered the mouth of the Danube River and headed toward the Izmail river port near the border with Romania.
Social media groups reported hearing air defence systems firing in the area near two Danube ports - Izmail and Reni.
The governor of southern Odesa region, Oleh Kiper, asked residents of Izmail district to take shelter at around 1:30 a.m. (2230 GMT) and cancelled the air raid alert one hour later.
Ukraine's Danube ports accounted for around a quarter of grain exports before Russia pulled out of a U.N.-backed deal to provide safe passage for the export of Ukrainian grain via the Black Sea. They have since become the main route out, with grain sent on barges to Romania's Black Sea port of Constanta for shipment onwards.
A Russian attack on the Izmail port sent global food prices higher in early August.
(Reporting by Maria Tsvetkova; Editing by Mark Porter and Stephen Coates)