A prominent US lawmaker has asked the small South Pacific island nation Tuvalu to stop reflagging Iranian oil tankers and warned its government of the risks of running afoul of US sanctions.

The European Union banned Iranian oil imports as well as providing insurance for vessels carrying Iranian oil on July 1, and the United States has new economic sanctions that have curbed Iranian oil imports by most other major nations.

Reflagging ships masks their ownership, which could make it easier for Iran to obtain insurance and financing for the cargoes, as well as find buyers for the shipments without attracting attention from the United States and European Union.

The National Iranian Tanker Company (NITC) changed the names and flags of many of its oil tankers ahead of the EU ban, part of sweeping economic measures aimed at pressuring Tehran to end its nuclear programme.

As many as 22 ships owned by the National Iranian Tanker Company have been registered in Tuvalu, said Howard Berman, the top Democrat on the House of Representatives Foreign Affairs Committee.

"This has the effect of assisting the Iranian regime in evading US and EU sanctions and generating additional revenues for its nuclear weapons program and its support for international terrorism," Berman said in a letter to Tuvalu Prime Minister Willy Telavi.

Berman said he believes reflagging the tankers could be deemed as "sanctionable activity" under US rules, and asked Telavi to cancel the registry of all NITC vessels.

"Given the close and cooperative relationship that our two governments now enjoy, it would be unfortunate if this action were permitted to stand," Berman said.

Officials from the country's permanent mission to the United Nations were not immediately available for comment.

Berman has worked with Democratic Senator Robert Menendez on legislation that would toughen US sanctions against the NITC. The US Congress could finalize those new sanctions this month.

Berman recently wrote to Tanzania's president about similar reports that the east African nation had reflagged Iranian tankers.

Last week, Tanzania's foreign minister said he would investigate and would strip the vessels of the country's flag if reports were true.

"If it is confirmed that the ships flying Tanzania's flag are indeed from Iran, we will take steps to deliberately obliterate the registration," Foreign Affairs Minister Bernard Membe said.

- Reuters