The U.S. House has voted to bar the U.S. government from purchasing heavy water from Iran in the future, undercutting President Barack Obama's nuclear pact with Tehran.
The House attached the prohibition to an essential funding bill for the U.S. Energy Department with a 251-168 vote.
A similar amendment was blocked by Democrats in the Senate last month shortly after the Obama administration completed an $8.6 million deal to buy 32 tons of heavy water from Iran. The White House threatened to veto the Senate measure.
The House amendment wouldn't reverse last month's purchase, but it would stop future purchases. However, there are no plans for further U.S. purchases of heavy water.
The heavy-water sales help Iran carry out its obligations under the nuclear deal. Iran has also sold heavy water to Russia.
Under the deal, Iran is allowed to use heavy water in its modified Arak nuclear reactor, but must sell any excess supply of both heavy water and enriched uranium on the international market.