Ukraine struck one of Russia’s largest ports with naval drones, disrupting shipments of grain and oil as fighting in the Black Sea intensified, threatening world food and energy markets.

The Wall Street Journal reports that the strike on August 4 came after Russia hit numerous port and grain-terminal targets along Ukraine’s Black Sea coast and along the Danube River, disrupting Ukraine’s ability to export grain.

The Russian Defense Ministry said it repelled the attack on the port at Novorossiysk, destroying two Ukrainian surface drones. Also late August 4, a Russian oil tanker was hit near the Kerch Strait, Russia’s Tass news agency reported, blaming the attack on Ukraine. Ukrainian officials, in keeping with their usual practice, didn’t publicly acknowledge responsibility for the attack and an army spokeswoman suggested it was a Russian provocation. However, one Ukrainian official, speaking privately, confirmed the attack was a Ukrainian operation.

The attack appears to be a Ukrainian attempt to impact Russia’s grain exports in the Black Sea. Russia also uses the port for exporting oil from its oil fields and in Kazakhstan to global markets.

Russian state broadcaster Vesti said traffic has been temporarily stopped at the port, though oil was still being pumped.