Bush signed the "instrument of ratification of the protocol additional to the agreement between the United States of America and the International Atomic Energy Agency (IAEA)" to implement such nuclear safeguards, a White House statement said.

The protocol was signed by the United States and the IAEA on June 12, 1998 and approved by the Senate on March 31, 2004.

Asked by AFP whether Tuesday's signing meant the additional protocol could now go into force, White House spokesman Gordon Johndroe said yes.

The additional protocol boosts the IAEA's ability to uncover secret nuclear weapons programs in non-nuclear weapons states by providing more information about global nuclear activities.

Although the United States is not bound to accept IAEA safeguards, it has allowed the UN nuclear watchdog to implement them at US nuclear sites for more than 40 years, an administration official said.

Some sites or activities are excluded from such monitoring however for national security reasons, the official added.

To date, 118 countries have signed an addition protocol with the IAEA and 89 countries have ratified it. Such a protocol steps up measures such as permitting IAEA inspections of nuclear sites.

The Bush administration has argued in favor of a universal adoption of an additional protocol, the administration official added.