But Iran has continued its program despite the sanctions. In February, in an attempt to move flagging negotiations forward, the world powers offered broader concessions to Iran, including letting it keep a limited amount of enriched uranium and suspend — but not fully close — a bunker-like nuclear facility near the holy city Qom. The world powers' offer, which also included removing some of the Western sanctions, was hailed by Iran as an important step forward in the process.
Few expect any major breakthroughs in the negotiations beginning this week until after Iran's presidential election in June.
Meanwhile, fighting in Syria has only intensified, and fears that Assad's forces used chemical weapons on rebel fighters in March brought the U.S. closer than ever to sending military aid to the opposition. Yet Obama has resisted pressures from foreign allies, Congress and his own advisers to arm the rebels or at least supply them with military equipment, or to use targeted airstrikes to destroy some of Assad's warplanes. The U.S. is helping train some former Syrian army soldiers — mostly Sunni and tribal Bedouins — in neighboring Jordan, which officials describe as non-lethal aid.
Part of Obama's reluctance, officials say, is the fear that U.S. weapons could end up in the hands of jihadists affiliated with al-Qaida. Of top concern is the Jabhat al-Nusra, a wing of the Islamic State of Iraq which, in turn, blames Iran for supporting the Shiite-led government in Baghdad.
"Since we are now looking more at a pending regime collapse in Damascus that has a strong potential to turn it into a launch pad for transnational jihadism, Washington is more interested in a negotiated settlement, which involves talking to Iran," said Kamran Bokhari, a Toronto-based expert on Mideast issues for the global intelligence company Stratfor.
Obama has been firm in his belief that Assad must go, and has predicted it will happen sooner than later. But he has been equally adamant that Iran must be stopped from acquiring nuclear weapons.
"A nuclear-armed Iran would be a threat to the region, a threat to the world and potentially an existential threat to Israel," Obama said at a March 20 news conference in Jerusalem, flanked by Israeli Prime Minister Benjamin Netanyahu. "And we agree on our goal. We do not have a policy of containment when it comes to a nuclear Iran."
Assad's fall would strip Iran of its closest ally in the volatile Mideast and perhaps spur the Islamic Republic to aggressively pursue a nuclear weapon as it faces further isolation. At the same time, it could encourage Tehran to make modest concessions on nuclear talks to relieve pressure from the West, said Gary Samore, who in February left the White House as Obama's coordinator for arms control and weapons of mass destruction and is now at the Kennedy School of Government at Harvard University.
"You can argue it either way, but in the end I think the collapse of Assad makes a nuclear deal more likely, because the Supreme Leader (Ayatollah Ali Khamenei) will feel more isolated, under greater pressure, more likely to make tactical concessions in order to relieve further isolation and pressure," Samore said Monday. "Of course, that is not going to change his fundamental interest in acquiring a nuclear weapons capability. I think it will confirm for him that the best way to defend himself against countries like the United States is to have that capacity."
Associated Press writer Brian Murphy in Dubai, United Arab Emirates, contributed to this report.
Lara Jakes has covered national security for The Associated Press since 2005 and is a former AP chief of bureau in Baghdad. Follow her on Twitter at: https://twitter.com/@larajakesAP
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