Bushehr nuclear reactor
Bushehr nuclear reactorReuters

Lawmakers are raising questions about the secret U.S. diplomacy with Iran that led to last month's nuclear breakthrough, The Associated Press (AP) reported on Wednesday.

Hours after the deal with Iran was reached in Geneva on November 24, it was revealed that the United States and Iran had secretly engaged in a series of high-level, face-to-face talks since at least this past March.

The secret talks were not revealed to the other countries negotiating with Iran until much later.

On Wednesday, Senator Bob Corker, who recently visited Oman where some of these talks took place and discussed the diplomatic back-channel with the country's foreign minister, asked one of the American officials involved to explain the process.

Puneet Talwar, President Barack Obama's senior Mideast adviser who has been nominated for a senior State Department post, responded by saying he met with an Iranian team in Oman in the summer of 2012 and then again in March 2013.

He said the talks only heated up after Iran's new president, Hassan Rouhani, took office in August.

"We then had an accelerating pace of discussions bilaterally with the Iranians," Talwar said at the Senate Foreign Relations Committee hearing, according to AP.

He added that the discussions, however, were constantly linked to public, parallel negotiations involving Iran and the U.S., Britain, China, France, Germany and Russia.

"It was made clear," Talwar said. "It focused exclusively on the nuclear issue, so there were no other side discussions underway. And it was merged, after the conversations gained traction," with the process involving the global powers.

Sen. Marco Rubio, R-Fla., made clear his disappointment at not being informed of the private diplomacy and, according to AP, asked Talwar if any member of Congress had been briefed. Talwar said he didn't believe so.

Rubio also asked whether other issues came up in the talks, such as human rights, Iran's support for Hezbollah and Hamas, and an alleged Iranian assassination plot against Saudi Arabia's ambassador in Washington.

Talwar said only that the fate of three detained Americans in Iran had been discussed.

According to the deal signed between Iran and the West, Iran’s nuclear program will be scaled back for a period of six months, while some of the sanctions that have crippled the Islamic Republic’s economy will be lifted.

The six-month period has not yet begun, and technical experts from Iran, world powers and the UN atomic watchdog started talks this week about when the implementation of the deal will start.

Some senators have expressed their objection to the deal with Iran and have been working hard on a new bill that would impose harsher sanctions on Iran.

The Obama administration, for its part, has waged an aggressive campaign to convince lawmakers to postpone passing new sanctions on Iran.

This campaign seemed to achieve a partial victory on Tuesday, when the Senate Banking Committee announced it will hold off on passing a new Iran sanctions bill.

At the same time, Sens. Chuck Schumer (D-N.Y.), Robert Menendez (D-N.J.) and Mark Kirk (R-Ill.) continue to pursue a bipartisan sanctions bill that would likely give a six-month window for further negotiations before taking effect.