Ukraine seized a foreign cargo ship on the Danube River and detained the captain on suspicion of helping Moscow export Ukrainian grain from Russian-occupied Crimea, officials said on Thursday.

Kyiv has accused Russia of trading stolen Ukrainian grain since the 2022 war began. Ship seizures, however, have been rare and shipping sources said they were concerned about possible retaliation at a critical time in the year as Ukraine grain exports reach a peak.

The Cameroon-flagged USKO MFU was presented with an arrest order and detained along with its Azeri captain after entering the waters of Reni, a Ukrainian port on the Danube where the river forms the border with Romania, prosecutors said.