WASHINGTON — As a first round of talks toward a long-term comprehensive nuclear agreement between the P5+1 states and Iran began Tuesday, critics warned that the Vienna negotiations could end in a deal – or a stalemate — that would leave Iran dangerously close to a nuclear weapon.

Mark Dubowitz, the executive director of the Foundation for the Defense of Democracies think tank, a strong sanctions proponent, warned that during the months of comprehensive negotiations, Iran would still have the capacity to hone technologies that would advance its ability to weaponize its stockpiles of enriched uranium. Research on delivery systems, warhead designs, nuclear triggers and ballistic missiles, he cautioned, could continue apace, unfettered, for months.

“Iran is gaining itself a minimum of six months to work on those elements, maybe longer,” he told The Times of Israel. “Iran has shown a willingness to compromise on the parts of its nuclear program that it has already perfected.”

In addition to weapons delivery systems, Dubowitz warned, Iran would be able to continue research and development on advanced centrifuges, although it is forbidden from installing any new centrifuges under the terms of an interim nuclear deal it reached with world powers in November 2013. Advanced centrifuges would enable Iran to use fewer centrifuges to enrich its uranium to near-weapons-grade levels.

In the face of the criticism, Obama administration officials expressed guarded optimism. Asked about the chances for a long-term deal with Iran, White House spokesman Jay Carney said Tuesday night that “because there is at least some prospect that Tehran might be willing to — in a verifiable, transparent way — convince the international community that it has forsaken pursuit of a nuclear weapon, we ought to do that through diplomatic means.”

While acknowledging that the threat of military force against Iran must remain “on the table,” he asserted that it “can’t be a first option.”

Carney told reporters that the administration is “obviously mindful of the fact that [the negotiations] may not result in an agreement, but because they present the opportunity, we have to take it.”

“Our view hasn’t changed in that we think that it is absolutely the right thing to do to test whether or not Tehran is serious about resolving this conflict diplomatically,” he explained.

Carney responded to critics who, like Dubowitz, have been complaining that the interim agreement didn’t curtail in a meaningful way Tehran’s ability to break out to a bomb.

The White House spokesman said that according to the terms of the interim agreement, Iran must address the UN Security Council resolutions related to its nuclear program before a comprehensive resolution can be reached. Referencing a 2010 Security Council resolution to that effect, Carney argued that the agreement obliged Iran to “deal with matters related to their ballistic missile program.” He said fulfillment by Iran of the obligations set down in the interim deal was a prerequisite to a comprehensive agreement.

A senior administration official speaking to reporters in Vienna in advance of the talks noted that the number of centrifuges allowed Tehran will be a major point of discussion in the negotiations toward a comprehensive solution. The official did not, however, discuss the trajectory of talks regarding new centrifuge technology in the comprehensive framework.

A Senate bill that would increase sanctions against Iran should the talks prove unsuccessful has been languishing in Congressional deep freeze, scant votes short of a veto-proof majority. Supporters of the bill say that it would have been one of Iran’s few incentives to reach an agreement rather than dissemble while continuing to build up the still-permissible aspects of its nuclear program.

AIPAC, which lobbied heavily for the bill, released a memo Tuesday in which it said that the final agreement with Iran must include the dismantlement – and not merely suspension – of Iran’s existing nuclear capabilities.

“Tehran must dramatically alter its position if the Vienna talks are to have any chance for success,” warned the pro-Israel lobbying group. It cited past quotes by US President Barack Obama, Secretary of State John Kerry and other top administration officials insisting that Iran must dismantle key aspects of its nuclear program, including the fortified underground uranium enrichment facility at Fordo and the Arak heavy water reactor, whose future plutonium byproduct could furnish a second route to the bomb.

While cynicism toward Iran’s intents has been running high in some circles, others in Washington were more optimistic about the renewed talks. On Friday, a bipartisan group of 104 members of the House of Representatives signed on to a letter congratulating the Obama administration for its efforts.

The members of Congress emphasized that the ongoing implementation of the interim agreement “increases the possibility of a comprehensive and verifiable international agreement.”

The letter reiterated the administration’s complaints regarding the new sanctions legislation, noting that although Congress “may be compelled to act” toward additional sanctions should talks break down, the representatives believe that while talks are ongoing “a bill or resolution that risks fracturing our international coalition or, worse yet, undermining our credibility in future negotiations and jeopardizing hard-won progress toward a verifiable final agreement, must be avoided.”