By Patricia Zengerle

WASHINGTON (Reuters) - The House of Representatives passed legislation on Wednesday to block the purchase of "heavy water" from Iran, defying President Barack Obama's veto threat a year after the announcement of the landmark Iran nuclear agreement.

The House passed the "No 2H2O from Iran Act" by 249-176, with support coming almost exclusively from Republicans, who hold a majority of seats in the chamber.

Every congressional Republican, and a few of Obama's fellow Democrats, opposed the nuclear deal between Iran and the United States and other world powers announced last July 14 in which Tehran agreed to curtail its nuclear program in exchange for relief from crippling international sanctions.

They argued that Obama was so eager to bolster his foreign policy legacy that he agreed to sanctions relief Iran did not deserve because it lied about its nuclear program, supported militant groups that attack U.S. allies and perpetrated human rights abuses.

Deal opponents have repeatedly introduced legislation that the administration sees as efforts to undermine the international agreement. Deal supporters said the agreement was the best way to defuse a dispute over Iran's nuclear program that threatened further Middle East destabilization.

Heavy water is a non-radioactive byproduct from making nuclear weapons and nuclear energy. The Obama administration said in April the United States would buy $8.6 million worth of heavy water from Iran, angering Republicans who called it a subsidy of the country's nuclear program.

The White House issued a threat on Monday to veto the bill, and two others the House is due to consider later this week.

Representative Ed Royce, chairman of the House Foreign Affairs Committee, denied that the House measure would violate the landmark pact. He said it would deter Iran from producing heavy water by making its sale more difficult.

Separately, two Senate Republicans and two Democrats said on Wednesday they would introduce their own Iran-related bill.

The measure introduced by Republicans Bob Corker, chairman of the Senate Foreign Relations Committee, and Marco Rubio and Democrats Robert Menendez and Joe Manchin would expand sanctions for Iran's ballistic missile development, sanction transfers of conventional weapons to or from Iran and extend the Iran Sanctions Act.

The Iran Sanctions Act, which imposed sanctions over Iran's missile development and support for terrorism, expires at the end of 2016. Lawmakers have been in discussions for months over the best way to address its renewal.

(Reporting by Patricia Zengerle; Editing by Peter Cooney)