US Secretary of State John Kerry will return to Switzerland for talks Thursday on a deal to restrain Iran's nuclear program, five days before a deadline to reach the outlines of an agreement.
Kerry's office said Monday he would meet his Iranian counterpart Mohammad Javad Zarif in Lausanne for the next stage in talks between the so-called "P5+1" world powers and Iran.
The delegates have in theory given themselves until the end of the month to agree the broad outlines of a political accord to govern a future final settlement of the stand-off.
Washington and its allies believe Iran's purportedly civilian nuclear power program is a cover for an alleged drive by the Islamic republic to develop atomic weapons.
This would, they fear, upset the already precarious balance of power in the Middle East, trigger an arms race and pose a threat to US allies like Israel and Saudi Arabia.
Tehran insists it has no desire for nuclear arms, but has agreed to discuss increased international controls on its facilities in return for looser economic sanctions.
President Barack Obama has made the quest for a deal the key plank in US non-proliferation policy, despite concerns at home and abroad that Tehran is simply playing for time.
- 'Aggressive politics' -
On Monday, more than 80 percent of the lawmakers in the US House of Representatives -- both Democrats and Republicans -- wrote to Obama to argue that any deal should constrain Iran for decades to come.
"Given Iran's decades of deception, negotiators must obtain maximum commitments to transparency by Iran," said the letter from the 367 representatives.
"Any inspection and verification regime must allow for short notice access to suspect locations, and verifiable constraints on Iran's nuclear program must last for decades."
For months now, Congress has been at loggerheads with the Obama administration, with members of the Republican majority criticizing a deal even before it is struck.
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Many Democrats are also skeptical and have joined their opponents in threatening to block any accord they feel does not sufficiently dismantle Iran's nuclear infrastructure.
Two weeks ago, Republican senators set off a furor by undermining Obama by warning Iran that he does not have the power to conclude a durable agreement without their backing.
Administration officials have swarmed the Congress to pressure lawmakers.
Last week, State Department number two Antony Blinken insisted any agreement would prevent Iran from building a weapon and that the easing of US sanctions would be gradual.
The Senate Foreign Affairs Committee has scheduled an April 14 meeting to consider a bill that would require consultation with Congress in the event an accord is reached.
The talks have not just raised concerns in Washington, however.
Israel's skepticism about any US accord with its old foe is well known, but Saudi Arabia is just as concerned.
On Monday, Saudi Foreign Minister Prince Saud al-Faisal said: "It is impossible that Iran should get undeserved deals."
Prince Saud called for guarantees that the program "does not turn into a nuclear weapon that could pose a threat to the region and the world, especially in view of Iran's aggressive politics in the region."
The minister accused Tehran of "continued meddling in the affairs of Arab countries and attempts to stoke sectarian conflicts in the region".
The long tense relations between Iran and Saudi Arabia, the foremost Shiite and Sunni Muslim powers in the Middle East, worsened when they took different sides in the Syrian civil war.
Tehran backs Syrian President Bashar al-Assad while Riyadh supports the Sunni rebels trying to topple him.
Iran also supports Shiite armed movements in Iraq and Lebanon, and has been accused of fomenting trouble in Yemen.