WASHINGTON — Iran’s unwillingness to move on its positions during recent rounds of nuclear negotiations indicates Tehran’s negotiators may be incapable of sealing a comprehensive agreement, veteran US diplomat Dennis Ross said Tuesday.

A day after nuclear talks with Iran were extended until July 2015 after the sides failed to come together following a year of intensive negotiations, Ross said that the US had demonstrated flexibility during the talks, including a willingness to back down on demands over the Arak heavy water facility and the Fordo enrichment facility, but that its positions were received by intransigence by the Iranian counterparts.

Ross, a former special adviser on the Persian Gulf for the US State Department who is currently a fellow at the Washington Institute for Near East Policy, did not play any official role in the talks. He did not detail how he knew the inside working of the negotiations, which have mostly been kept under wraps.

The former diplomat said that Iranian negotiators were either unwilling or incapable of budging from a series of red lines.

Iran, Ross said, would not roll back centrifuge programs for uranium enrichment to the levels that the P5+1 members hoped, and “would not budge on the demands that sanctions be removed immediately” upon the achievement of a comprehensive agreement.

However, he said current Secretary of State John Kerry’s upcoming Congressional briefing on the talks would have to convince legislators that progress had been made, and that there was a reasonable chance of reaching an acceptable agreement by the end of the seven-month extension.

“The fact that the Iranians did not take advantage of US flexibility raises questions in my mind as to whether they are really capable of doing a deal,” Ross warned in a conference call with Jewish Federations of North America, arguing that Iranian negotiators’ hands may be tied by the anti-American ideology of the Islamic Republic’s religious leadership and specifically that of Supreme Leader Ali Khamenei.

Khamenei has maintained strident anti-American rhetoric even as President Hassan Rouhani has offered a more moderate stance toward the West.

Ross noted areas in which US negotiators showed flexibility toward Tehran. He said they were apparently willing to back down from demands that the Arak plutonium enrichment facility be refitted from a heavy water to a light water plant – instead suggesting that the US was prepared to allow it to remain a modified heavy water facility.

While in the beginning, he said, the US wanted to “shutter” the secret underground uranium enrichment facility at Fordo, the US and P5+1 partners moved toward suggesting that the facility be allowed to remain open as a “research facility.”

Kerry is expected to brief the Republican-controlled Congress in coming days regarding the talks’ extension. The briefing will be classified and closed-door, which Ross said indicates that Kerry is likely to delineate areas where he feels gaps were narrowed during the 10 months of talks under the interim deal reached a year ago.

Ross said he sees a productive role for Congress in the coming months, suggesting that legislators could work with the Obama administration to press the Iranians to discuss possible military applications of its nuclear program, including coming clean about previous military development that Iran has pursued in recent years.

P5+1 negotiators have said that Iran’s intercontinental ballistic missile development program is not on the negotiating table under the current framework, but critics have stressed that nuclear weapons delivery systems are a critical part of Iran’s nuclear project.

On Sunday, Prime Minister Benjamin Netanyahu pointed to the ICBMs as proof of Iran’s desire to produce nuclear weapons, in lobbying for negotiators not to sign “a bad deal.”

Iran insists its program is peaceful, a notion doubted by most Western countries.

Congress could also push, Ross said, for assurances from the administration regarding the consequences should Iran “cheat” on any final agreement. Ross included among those actions a preliminary Congressional approval for the use of military force, to be applied in instances in which Iran violates terms of the comprehensive agreement.

Such guarantees, he said “would reassure those on [Capitol] Hill who have concerns about the vulnerabilities of the agreement.”

With a Republican-controlled Congress that is likely to take a more cautious approach toward concessions to Iran, Ross said delineating progress is a key element in securing any Congressional support for the continued talks.

Opponents of a continued negotiation, he said, will need explanations from Kerry as to why there is still any reason to negotiate if 10 months of talks have so far failed to yield a comprehensive nuclear agreement.

Ross suggested that conditions have to shift in order for Iran to be likely to accept any agreement that takes P5+1 concerns into consideration.

“If there is going to be a deal, the Iranians must have a perception that they don’t have today – that they want a deal more than we do,” he said. “They need to see that the conflict with ISIS doesn’t make us need them more than they need us,” he said, adding that the negotiators need to “make clear to the Iranians that there is a high price to pay” if a deal is not struck.

“I don’t believe that the Iranians have tempered anything they are doing in the region because of the nuclear negotiations, but I fear that we have tempered our behavior in the region because we are worried that it will impact the talks,” Ross said, citing US engagement in the Syrian civil war as an example.

“We want them to see that failure of diplomacy on the nuclear issue is something they have something to lose from,” he asserted.

At the same time, Ross acknowledged that unless Iran is seen to resume a race to a nuclear bomb, “the possibility of military actions by US and or Israel is low” even if no deal is reached.

He emphasized a likelihood that even if no deal is struck, Iran will continue to abide by its obligations under the interim arrangement, with the current eased sanctions framework remaining in place.