Traces of enriched uranium have been found on smelted aluminum tubing provided to the United States by North Korea in its effort to prove it was not operating a secret nuclear program, The Washington Post reported.

Citing U.S. and diplomatic officials, the Post said the discovery by U.S. scientists apparently contradicts North Korea's claim that its acquisition of thousands of aluminum tubes were for conventional purposes rather than a nuclear program. Such tubes could be used in the process of converting hot uranium gas into fuel for nuclear weapons, according to U.S. officials.

The U.S. did not want to reveal the discovery of the uranium traces because it could expose intelligence methods and complicate diplomatic efforts aimed at North Korea, the Post said in a story posted on its Web site late Thursday. The office of the director of national intelligence and the State Department declined to comment on the discovery, the newspaper said.

The tubing could have been contaminated by exposure to other equipment rather than by an active enrichment program, the Post said, citing unidentified officials.

North Korea is expected to issue a declaration by the end of the month in which it outlines all its nuclear programs. Under an agreement with China, Japan, Russia, South Korea and the United States, North Korea has begun disabling its main nuclear facilities.

Secretary of State Condoleezza Rice told The Associated Press last week that the Bush administration was not ready to "engage broadly" with North Korea until its leadership ended all aspects of its nuclear weapons program.

North Korea and Iran "are clearly still states about which there are significant proliferation concerns," Rice said.