An Estonian-owned cargo ship has sunk off the coast of Ukraine after encountering a sea mine, with several crew missing.

Four members of the crew were still unaccounted for as of Thursday morning after the cargo ship Helt experienced an explosion and sunk near the port of Odessa.

Besides the missing crew members, two others escaped in a life raft, said Igor Ilves, managing director of Estonia’s Vista Shipping Agency, the Mirror reported.

Ilves said that the 1,500 ton ship, which was flying under the flag of Panama, hit a mine off the port.

“The vessel has finally sunk,” Ilves said. “Two of the crew are in a raft on the water and four others are missing. I don't know where they are at the moment.”

The Russian Black Seat Fleet has been accused of using the Helt and other ships as human shields as it invaded Ukraine, according to the Daily Mail. The Russian Navy also compelled the Helt to travel through a treacherous area of the Black sea in order to cover up the whereabouts of its war fleet.

Ukraine's armed forces said in a statement that “for the safety of civil navigation, the Navy of the Armed Forces of Ukraine closed navigation in the north-western part of the Black Sea.”

They added: “At the same time, the Russian Black Sea Fleet continues the tactics of Russian ground forces trying to hide behind civilian lives.”

Estonia is a member of NATO. The incident sparked worry that the current war in Ukraine could expand into other regions of Europe.

Commercial shipping to Black Sea ports of Ukraine has been largely suspended as insurance premiums have become costly while the war continues.