By Patricia Zengerle and Timothy Gardner
WASHINGTON, Jan 14 (Reuters) - Senate Majority Leader Harry Reid made clear on Tuesday he has no immediate plan to allow a vote on a bill that would slap new sanctions on Iran over its nuclear program, even as backers promised to keep up their efforts to win more support.
Fifty-nine of the 100 senators - including 16 of President Barack Obama's fellow Democrats - support the bill, despite Obama's warning that its passage could jeopardize delicate international negotiations to curb Iran's nuclear program.
New Jersey Senator Robert Menendez, the Democratic chairman of the Senate Foreign Relations Committee, is a lead sponsor of the measure, which has caused friction between the White House and some Democrats in Congress.
Iran warned that it would back away from the negotiations if any new sanctions were passed. Obama promised to veto the measure if it passes both the Senate and House of Representatives.
Asked when he might allow a Senate vote on the legislation, Reid acknowledged the support, but also noted that 10 Democratic committee chairs had written to him opposing it.
"Let's see how this plays out," he told reporters.
"The legislative process is working forward here. I am going to sit and be as fair an umpire as I can," Reid, a Nevada Democrat, said.
Kentucky Senator Mitch McConnell, the top Republican in the Senate, told reporters he believed the vote should take place, and suggested that backers might be able to muster the 67 votes necessary to override a presidential veto.
"We're going to continue to continue to press in order to allow a vote on an issue that obviously enjoys the support of a very large bipartisan majority here in the Senate," he said.
A weekend agreement to begin implementing a temporary nuclear deal between Iran and world powers on Jan. 20 helped reinforce the argument that the sanctions bill should not go ahead while negotiations continue, even though it would not actually impose the new restrictions unless Iran failed to comply with the interim agreement.
'SECRET SIDE DEAL'
But backers of the bill said on Tuesday reports that the Obama administration has negotiated a "secret side deal" with Iran on the future of its nuclear program underscored the importance of doing everything possible to clamp down on Tehran.
"We call on the Obama Administration to clarify this situation immediately and ensure that members of Congress are fully and promptly informed about its nuclear diplomacy with Iran," Republican Senators John McCain and Lindsey Graham, both sanctions bill co-sponsors, said in a statement.
The White House said there is no secret agreement, merely that the documentation associated with the interim agreement has not yet been released.
"We will make the text available to the Congress and the public, but we must work with the parties on when and in what format the information will be released. And we hope to do that soon," White House spokesman Jay Carney told reporters during a daily news briefing.
Olli Heinonen, a former deputy director general at the U.N.'s International Atomic Energy Agency, said it was not surprising that a document from the agreement was not immediately released to the public.
"Have we had these kinds of understandings before? Actually I think so," Heinonen told reporters in a teleconference on Tuesday hosted by The Israel Project advocacy group.
He said the document probably has two parts; half on the undertakings at Iran's nuclear installations and centrifuges, and the other on the timing of sanctions relief.
The 1994 agreed framework between Washington and North Korea over the Asian country's nuclear power plants contained "certain understandings" that were never officially disclosed to the public, Heinonen said. (Additional reporting by Timothy Gardner, Steve Holland and Richard Cowan; editing by Meredith Mazzilli)