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But Iranian leaders dropped the demand Tuesday. Foreign Minister Mohammad Javad Zarif told the semi-official ISNA news agency that while enrichment itself was a “nonnegotiable” right, Tehran does not see any “necessity for its recognition as a right.”

Israel has spent recent days ramping up pressure on Washington to accept nothing less than a total halt in enrichment before any sanctions on the country are lifted, particularly in light of fears that Iran simply may be using the negotiations to stall for time while continuing to secretly enrich uranium for a nuclear bomb.

Such fears fuel deep skepticism about the negotiations among some in Washington. Lawmakers from both parties worry that the Obama administration may be ceding leverage to Iran without taking firm action. Hours after the president’s meeting with congressional leaders, a bipartisan group of senators reiterated some of those concerns.

“While the interim agreement may suggest that Iran could be willing temporarily to slow its pursuit of a nuclear weapons capability, it could also allow Iran to continue making some progress toward that end under the cover of negotiations. This does not give us confidence that Iran is prepared to abandon unambiguously its nuclear weapons pursuit altogether, as it must,” reads a part of the letter, signed by Democratic Sens. Charles E. Schumer of New York, Robert Menendez of New Jersey and Robert P. Casey Jr. of Pennsylvania.

The Republican signatories were Sens. Lindsey Graham of South Carolina, John McCain of Arizona and Susan M. Collins of Maine.

Sen. Bob Corker, Tennessee Republican and his party’s ranking member on the Senate Foreign Relations Committee, emerged from Tuesday’s meeting with the president saying some lawmakers remain “unsatisfied” with the White House’s approach.

He said “nobody knows” whether the two sides will be able to reach an agreement in Geneva this week — something the president reiterated during his remarks later in the day.

Some analysts said that the Obama administration appeared to be seizing on the terrorist attack in Beirut as an opportunity to send a message to Iran that whether or not a breakthrough is achieved in the nuclear talks, Washington and Tehran could work together on other fronts.

Mr. Kerry’s condolence note “reinforces a message that President Obama has tried to convey for five years now, that the U.S. has a problem with the Iranian nuclear program, a problem with some Iranian policies, but not with Iran per se, and that when these problems are resolved, other issues can be addressed more collaboratively,” Mr. Parsi said.

Others dismissed the notion that Washington and Iran could see eye to eye when it comes to the threat of al Qaeda attacks.

“I would resist the narrative that both Iran and the U.S. face a common threat, because Iran certainly hasn’t treated it like a common threat,” said James Phillips, a senior research fellow for Middle Eastern affairs at the Heritage Foundation.

Despite the sectarian difference between al Qaeda — a Sunni organization — and Iran’s overwhelmingly Shiite society, Mr. Phillips said, Iran supported al Qaeda activities stemming back to the early 1990s when “al Qaeda members were trained in Iran according to the 9/11 report.”

Iran helped to create the Frankenstein monster that al Qaeda has turned into,” he said.

Either way, most analysts agree that the attack in Beirut may be an indication that al Qaeda-linked groups fighting in Syria against the regime of Bashar Assad are having rising tensions against Hezbollah, the Lebanon-based Shiite militant organization with close ties to both Iran and Syria.

The midmorning explosions outside the Iranian Embassy hit the neighborhood of Janah, a Hezbollah stronghold and home to several embassies and upscale apartments, leaving bodies and pools of blood on the glass-strewn street amid burning cars.

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