WASHINGTON - The United States warned the maritime industry on Thursday that Iranian ships were still using other country's flags in an attempt to evade Western sanctions against the country.

Sierra Leone is the latest country to revoke its flag for a vessel controlled by the Islamic Republic of Iran Shipping Lines (IRISL), which is on the US blacklist for its involvement in Iran's weapons program.

The United States and the European Union have implemented tough sanctions against Iran in an attempt to stop the country from acquiring nuclear weapons.

The US penalties forced Iran's major trading partners to reduce their Iranian crude imports. The EU enacted a ban from July 1 on insuring ships carrying Iranian oil that has also crimped Tehran's oil exports.

The United States said port authorities had to be on the look out for fabricated vessel registration and phony flags.

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