WASHINGTON (AP) :

The Trump administration hit more than 50 vessels, shipping companies and trade businesses with sanctions Wednesday in its latest bid to pressure North Korea over its nuclear programme.

The administration billed it as the largest installment of North Korean economic restrictions to date. President Donald Trump went further, declaring in a speech yesterday at the Conservative Political Action Conference that it was "the heaviest sanctions ever imposed on a country before".

While the number of companies from North Korea and other nations was high, the economic impact is unclear. It was certain to be less than previous US measures that targeted large banks and business networks in China and Russia that deal with the North.

The United Nations Security Council has imposed three sets of sanctions on North Korea in the past year to deprive it of revenue and resources for its nuclear and ballistic missile development. Those weapons pose an emerging threat to the US mainland. Washington is particularly concerned about exports of North Korean coal that are prohibited by the UN sanctions and ship-to-ship transfers of imported oil and petroleum products.

The Treasury Department said it was barring US business transactions with nine inter-national shipping companies from China, Hong Kong, Singapore and Panama, and nine of their vessels. It also blacklisted 16 North shipping companies and 19 of their North Korean-flagged vessels.