Customs officials said Thursday that at least 300 trucks exporting goods from Ukraine were lined up at the border, as tensions between the two countries escalate.
Ukrainian officials blamed the delays on new customs procedures introduced unexpectedly by Russia, while Russia maintained that the changes had been widely reported in advance.
Kiev's plans to develop deeper political and economic ties with the European Union have provoked anger in the Kremlin, which has been pushing for Ukraine to join a Moscow-led Customs Union.
Lines began forming at several crossing points between Russia and Ukraine on Thursday morning, and did not lessen during the day, a spokesman for Russia's Customs Service said.