At least two countries have struck cargo vessels off their registries in recent weeks after the ships were involved in transporting grains from Crimea to Syria, maritime officials told Reuters.

Crimea has been under Western sanctions since it was annexed from Ukraine by Russia in 2014 and has been frozen out of many export markets. That leaves Syria, which has also been targeted by sanctions, as an ideal trading partner for Crimea, given that the latter is open to receiving goods such as olive oil in exchange for wheat, Reuters said.

Such trade is dependent on continued access to a limited pool of ships that are willing to transport cargoes despite the sanctions restrictions.

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Reuters first reported last month that Crimea has stepped up grain exports to Syria over the past year in an arrangement that gives the Black Sea peninsula an outlet for its surplus crops and provides Syria's President Bashar al-Assad with a reliable source of wheat.

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Ukraine says shipments from Crimea violate Western sanctions and has complained to the United Nations' London-based shipping agency the International Maritime Organization (IMO) and asked member states to de-list any vessels involved.

Ocean-going ships are required to register with a national authority, known as a flag state, which sets out regulatory obligations that every vessel must abide by.

Ship-tracking and port data showed that ageing cargo ships have been used to transport cargoes from the Crimean peninsula to Syria, including the Crystal Galaxy and Snow White, which have been actively involved in the trade.

Footage filmed by Reuters in June showed the Snow White loading grain in Crimea, bound for Syria.

Tanzania confirmed that the Crystal Galaxy had been de-registered, adding in a June 12 statement that a number of vessels were continuing to use the Tanzanian flag fraudulently.

Sierra Leone's maritime administration confirmed separately that both the Crystal Galaxy and the Snow White had subsequently been cancelled from its flag registry. The Snow White's owner, Eldora Shipping Company, and the Crystal Galaxy's, Crystal Galaxy Shipping Co, did not respond to requests for comment.

Natalia Galibarenko, Ukraine's permanent representative to the IMO, told Reuters that many flag states had "duly fulfilled their obligations" and de-listed ships involved in such trade.

"The Ukrainian side will continue to monitor the situation closely to ensure compliance by all ship owners, ship operators and ship masters," she said.

Galibarenko added that ships involved in such activity were switching off tracking transponders when approaching waters close to Crimean ports.

"As a result, the safety and security of navigation in the area have been severely compromised," she said.

Some of the other ships involved in the transport of grain cargoes have included Syrian-flagged vessels Souria, Laodicea and Finikia, which were added to a U.S. blacklist in 2015.