WASHINGTON, Aug. 18 -- The wooden crates moving across the Caspian Sea are carrying more than trade goods. Russia has been routing explosives, drone components and battlefield ammunition to Iran through the inland sea, in a transfer operation that Western officials say is rebuilding the military arsenal Tehran depleted fighting the United States and Israel this year.

A European government document obtained by NBC News, confirmed by a Western official, describes shipments that include trinitrotoluene, the explosive compound known as TNT, alongside drone parts and ammunition bound for Iran. The material represents exactly what Tehran needs to reconstitute the capabilities that American and Israeli strikes spent months targeting in the Iran...