Key backers of a US Senate bill that would intensify sanctions against Iran said Tuesday they were worried by reports that the White House had negotiated a “secret side deal” with the Islamic Republic over its nuclear program, and urged the Obama administration to come clean to lawmakers on the substance of an interim agreement reached in Geneva.

“Iran’s chief nuclear negotiator has claimed that, under this possible agreement, Iran will be permitted to keep all of its nuclear facilities open, continue its enrichment of uranium, and maintain and even expand its nuclear research, including into next-generation centrifuges,” Republican Senators John McCain and Lindsey Graham said in a statement.

“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,” they added, stressing the potential need for more sanctions.

The White House denied claims of any secret deal, saying the release of information on an interim agreement would have to be coordinated.

“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.

More than half the Senate has signed on to the bill to increase sanctions. But in a sign of the so-far successful effort by the White House to keep the bill from reaching a veto-busting 67 supporters, only 16 Democrats are on board.

The number of senators cosponsoring the bill, introduced by Sens. Mark Kirk (R-Illinois) and Robert Menendez (D-New Jersey), reached 59 this week, up from just 33 before the Christmas holiday break.

Notably only one of the 25 who signed up in recent days — Sen. Michael Bennet — is a Democrat, a sign of intense White House lobbying among Democrats to oppose the bill.

Backers of the bill say it would strengthen the US hand at the negotiations. But President Obama has said he would veto the bill because it could upend talks now underway between the major powers and Iran aimed at keeping the Islamic Republic from obtaining a nuclear bomb. A similar bill passed this summer by the US House of Representatives had a veto-proof majority.

On Thursday, the White House said backers of the bill should be upfront about the fact that it puts the United States on the path to war.

“If certain members of Congress want the United States to take military action, they should be up front with the American public and say so,” Bernadette Meehan, the National Security Council spokeswoman, said in a statement posted by The Huffington Post. “Otherwise, it’s not clear why any member of Congress would support a bill that possibly closes the door on diplomacy and makes it more likely that the United States will have to choose between military options or allowing Iran’s nuclear program to proceed.”

A number of pro-Israel groups, led by the American Israel Public Affairs Committee, are leading a full-court press for the bill’s passage, with prominent Jewish leaders in a number of states making calls and writing letters to holdouts. Dovish Jewish groups such as J Street and Americans for Peace Now oppose the bill.

The bill would expand sanctions in part by broadening existing definitions targeting energy and banking sectors to all “strategic sectors,” including engineering, mining and construction. It would also tighten the definition of entities eligible for exceptions and broaden the definition of targeted individuals who assist Iran in evading sanctions.