The United States coastguard says it'll automatically deny entry to any ship which has recently docked in Nauru.
Nauru is one of seven countries, with most others in Africa, which the US coastguard believes has failed to meet international maritime security standards brought in last year.
Reuters news agency reports that US officials will now board and screen all ships which have recently docked in Nauru before allowing them to enter US ports.
The Coast Guard says that from the 23rd of May, all ships which have docked in Nauru, on one of their last five stops, will automatically be denied entry to the United States.
Security boardings at sea, or a denial of entry to the United States, can trigger extremely costly delays, and perishable cargo can be lost completely.