By Timothy Gardner and Patricia Zengerle
WASHINGTON (Reuters) - More than a majority of U.S. senators support a bill to impose new sanctions on Iran should the Islamic Republic break an agreement to curb its nuclear program, aides said on Thursday, but there was no plan yet to debate the measure.
The White House has threatened to veto the legislation, and Iran says last November's nuclear deal struck in Geneva would be dead if the U.S. Congress imposes new sanctions.
The "Nuclear Weapon Free Iran Act" is now supported by at least 54 senators in the 100-member chamber, according to a congressional record, with six senators joining on Wednesday. A Senate aide said two more joined on Thursday, to bring the total to 56.
It is uncertain whether the bill will be introduced on the Senate floor and whether backers can win the two-thirds majority to overcome a veto by President Barack Obama.
A senior Senate Democratic aide said there were no plans yet for advancing the bill to the Senate floor, despite the growing list of co-sponsors.
The bill would also place sanctions on Iran if it does not agree to a comprehensive deal later this year or next. The United States and five other world powers agreed to a six-month interim deal with Iran in Geneva in November, that can be extended to a year.
Under last year's interim agreement, Iran will get access to billions of dollars worth of funds that had been cut off by sanctions in return for limiting enrichment of uranium.
At least 15 Democrats support the bill introduced in December by Robert Menendez, the chairman of the Foreign Relations Committee, and Mark Kirk, an Illinois Republican.
The American Israel Public Affairs Committee, an advocacy group with strong ties to many lawmakers, has said it supports new sanctions that would take effect if Iran violates the interim pact or does not agree to an "acceptable" comprehensive deal.
The bill seeks to cut Iran's oil exports to zero two years after implementation. It also puts limits on the Obama administration's ability to waive sanctions.
There is tough resistance to the measure by many other Democrats, however. A bloc of 10 Democratic senators, all leaders of committees, sent a letter to Senate Majority Leader Harry Reid last month expressing their opposition to the bill.
The aide said more support could come soon from the bloc of Democrats. "At least two that I know of are inching toward public support for the bill," the aide said on condition of anonymity given the sensitivity of the talks.
There has been no public indication from any of the 10 that they were leaning toward shifting position on the issue.
Nine senior foreign policy experts, including Ryan Crocker, a former ambassador to Israel and the United Nations, this week urged Menendez and Kirk to not pass the new sanctions, saying they could potentially move the United States closer to war.
(Additional reporting by Richard Cowan; editing by Alistair Bell and Rosalind Russell)