Senators urge Obama to push for strict Iran nuclear curbs deal

WASHINGTON (Reuters) - Twenty-three U.S. senators kept the spotlight on Iran nuclear negotiations on Saturday with a letter to President Barack Obama urging that he stand firm, after a second round of talks wound up in Vienna.

The letter from Democratic senators and one independent, was identical to one sent to Obama earlier this week by the House of Representatives, asking that he insist on a final agreement in which Iran would not be able to build or buy a nuclear weapon.

The House letter was signed by 395 of the 435 members of the chamber and was sent as Iran and six world powers met to persuade Iran to scale back its contested nuclear activities.

The meeting in Vienna was the second in a series that the six nations - United States, China, Russia, Germany, France and UK - hope will produce a verifiable settlement, ensuring that Iran's nuclear program is oriented to peaceful purposes only.

The 23 senators said they embraced Obama's two-track approach twinning sanctions against Tehran with negotiations, but urged strict procedures of transparency and verification to ensure Iran does not obtain a nuclear weapon.

The U.S. Congress has long taken a harder line on Iran than the White House, but Saturday's letter was an indication of how sensitive the issue is, even among members of the same party.

Many in this group of senators, including Carl Levin, whose office released Saturday's letter, did not sign a letter sent earlier this week from 83 of their colleagues.

That letter, spearheaded by Democrat Robert Menendez, took a more aggressive stance, urging Obama to insist that any final agreement state that Iran "has no inherent right to enrichment under the Nuclear Non-Proliferation Treaty."

That would be a non-starter for Iran, which cites a right under the NPT to produce nuclear energy for civilian purposes.

Both the U.S. and Iranian delegations - the two pivotal players in the negotiations - face intense pressure from hawkish critics back home.

(Writing by Doina Chiacu; Editing by Gunna Dickson)