US Secretary of State John Kerry is expected to face tough questioning at a Senate committee hearing as he defends a nuclear deal agreed with Iran.
The US Congress has two months to review the deal, which is opposed by Republicans, who control both Houses.
Senators on the Foreign Relations Committee are expected to want more detail on the provision of inspections and lifting of sanctions.
Many Republicans say last week’s accord conceded too much to Tehran.
Opponents within Congress could reject the agreement, although President Obama could veto their decision.
Negotiations between Iran and six world powers – the US, UK, France, China, Russia and Germany – began in 2006.
The powers suspect Iran of pursuing a clandestine nuclear weapons programme, but Tehran has always insisted its nuclear ambitions are for peaceful purposes.
As part of the deal, Iran has agreed to rein in its nuclear activity and accept a mechanism for inspections by nuclear officials in return for the lifting of international sanctions.
But those sanctions cannot be lifted while Congress is discussing the deal – until 17 September at the earliest.
Thursday’s meeting gives Mr Kerry – along with Energy Secretary Ernest Moniz and Treasury Secretary Jack Lew – the chance to make a case to lawmakers to pass the deal.
But the three cabinet officials are likely to encounter a great deal of scepticism.
Could US Congress torpedo the deal?
- Congress has 60 days to review the agreement
- During that time, President Obama cannot lift the sanctions Congress has imposed on Iran
- Congress can reject the deal, and keep the sanctions in place, but Mr Obama can veto that
- It would need a two-thirds majority to overturn the veto, which is unlikely
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House Speaker John Boehner has already vowed Republicans would “do everything possible to stop” the agreement.
Senate Republican leader Mitch McConnell said: “It’s always the administration, not Congress, that carries the burden of proof in a debate of this nature.
“And it seems the administration today has a long way to go with Democrats and Republicans alike”.
Bob Corker, the Republican chairman of the foreign relations committee, said he had a number of concerns – in particular the amount of “leverage” that would shift to Iran once sanctions eased – but that he had not yet decided how to vote.
A number of other Democrats have said they do not know how they will vote, including Ben Cardin, the committee’s most senior Democrat, who said he had “many areas of concern”.