Washington, fearing an attack or infiltration by al-Qaeda from the sea, has vowed to police the new United Nations codes strictly by turning away ships that are not security-certified or delaying ones that have called at "contaminated ports."

Adm. Thomas Collins, commandant of the US Coast Guard, told Reuters the agency had denied entry to six out of 265 ships arriving in the United States on Thursday for failing to have valid new security certificates.

He did not say what flags the six ships carried. He said none of them were oil tankers, but rather tended to be "freight-type vessels." Collins gave no further details on the ships, but said the tightened security regime had not had a significant impact on the flow of trade.

A US Coast Guard official said earlier the agency had ordered Bolivian-flagged cargo ship Dahomey Express to leave US waters because it lacked the new security certificates. There was no immediate information on the other five vessels.

The codes, signed by 147 governments, require ports, stevedoring companies and owners of ships larger than 500 tons to draw up plans for responding to a terror threat, implement tighter security around facilities and train staff.

The global shipping community is watching US enforcement efforts to check the effectiveness of the codes.

On the eve of the July 1 deadline to implement the new anti-terrorism measures, only about half of the world's ports and 53 percent of global shipping had complied with the International Ship and Port Facility Security code, according to the UN's International Maritime Organisation, its chief architect.