A State Department official says the U.S. does not know the precise location of tons of low-enriched uranium shipped out of Iran on a Russian vessel under the landmark nuclear agreement.

Testifying Thursday, Ambassador Stephen Mull tells the House Foreign Affairs Committee the stockpile is a Russian custody issue.

Critics of the nuclear deal seized on the shipment's status to show the agreement's flaws. New Jersey GOP congressman Chris Smith says it's "outrageous and unbelievable" that Russia is being trusted to be the repository for such sensitive material. Russia is a close ally of Iran.

The low-enriched uranium is suitable mainly for generating nuclear power and needs substantial further enrichment for use in the core of a nuclear warhead.

Mull says he's confident the material will be controlled properly.