From the article:

Ukraine at the weekend said its drones had struck two Russian vessels carrying military cargoes in the Caspian, the world’s largest inland body of water. It asserted that both had left Iran for a Russian port.

In a post on X Saturday, Ukrainian President Volodymyr Zelensky said the targets included “ships that were involved in transporting military cargo from Iran, as well as a warship,” subsequently identified by Ukraine’s security service as a Russian missile boat.

[...]

Russian grain and steel are among the cargoes that cross the Caspian. Iran has developed several significant ports on the sea, among them Anzali. Oil and gas from Kazakhstan are transported from the Caspian to a hub on the Black Sea.

For Russia, the Caspian is an important conduit to central Asia and Iran – and a critical route in its expanding strategic partnership with Tehran, one that has a military component as well as Russia’s role in Iran’s civilian nuclear program and oil exploration in Iran.

“The recent blockade by the US (of Iranian ports) has underlined the strategic value of the Caspian Sea route” says Khajehpour. It is also less vulnerable to international sanctions because it is landlocked and largely controlled by Iran, Russia and neighboring states beyond Western naval reach.

[...]

Russia received its first batches of Iranian Shahed attack drones via the Caspian early in its war with Ukraine and has since produced new versions of the drones in plants on Russian soil.

The Caspian route linking Iran’s northern ports to the Russian port of Astrakhan has been targeted by Ukraine before. Last August a cargo ship – the Port Olya 4 – was destroyed while reportedly loaded with Shahed-136 drone components and Iranian munitions.