Washington: US Secretary of State John Kerry on Thursday mounted a furious counterattack against critics of the Iran nuclear deal, telling skeptical lawmakers it would be fantasy to think the United States could simply ‘bomb away’ Tehran’s atomic know-how.

Testifying publicly before Congress for the first time since world powers reached the landmark accord with Iran last week, America’s top diplomat was confronted head-on by Republican accusations that Iranian negotiators had ‘fleeced’ and ‘bamboozled’ him.

The vitriolic exchanges at the Senate Foreign Relations Committee, which Kerry once chaired, reflected a hardening of positions as Congress opened a 60-day review of the deal considered crucial to its fate.

Iranian hardliners are also trying to undermine the pact, which US ally Israel calls a dire security threat.

Kerry insisted critics of the agreement, which curbs Iran’s nuclear program in return for sanctions relief, are pushing an alternative he dismissed as a ‘sort of unicorn arrangement involving Iran’s complete capitulation.’

‘The fact is that Iran now has extensive experience with nuclear fuel cycle technology,’ the former senator said. ‘We can’t bomb that knowledge away. Nor can we sanction that knowledge away.’

On crutches from a cycling accident, Kerry entered the hearing room to cheers from the anti-war group Code Pink.

Kerry said that if Congress rejects the accord, ‘the result will be the United States of America walking away from every one of the restrictions we have achieved, and a great big green light for Iran to double the pace of its uranium enrichment.’

‘We will have squandered the best chance we have to solve this problem through peaceful means,’ he said.

The 4-1/2-hour-long hearing was part of an intense Obama administration push to convince Democrats in particular to back the deal. Treasury Secretary Jack Lew and Energy Secretary Ernest Moniz also testified.

The three cabinet secretaries briefed the full House of Representatives and Senate behind closed doors on Wednesday and met privately with House Democrats after Thursday’s hearing.

Other senior administration officials, including President Barack Obama, have also been talking to undecided lawmakers. About a dozen met with him at the White House on Thursday.

Opening the hearing on a contentious note, the committee’s Republican chairman, Bob Corker, criticized Kerry for the terms he negotiated. ‘I believe that you’ve been fleeced,’ he said.

Another Republican, Jim Risch, said he had been ‘bamboozled.’

Democratic Senator Barbara Boxer called such accusations ‘disrespectful and insulting.’

Corker chided Kerry and other administration officials for contending that the only alternative to the accord would be more war in the Middle East, saying the real alternative would be a better deal.

Senator Marco Rubio faulted Obama for rewarding Iran for ‘its atrocious human rights record.’

‘This is a deal whose survival is not guaranteed beyond the current term of the president,’ said Rubio, a 2016 Republican presidential candidate.

Senator Ben Cardin, the top Democrat on the committee, said he has not yet decided how he would vote but that he felt ‘our negotiators got an awful lot.’

Under a bill Obama reluctantly signed into law in May, Congress has until Sept. 17 to approve or reject the agreement. Republicans hold majorities in both houses of Congress, and many have come out strongly against the pact, which they say will empower Iran and threaten Israel.

Obama, who could boost his presidential legacy from his diplomatic outreach to Iran, needs his fellow Democrats.

If a ‘disapproval’ resolution passes and survives Obama’s veto, he would be unable to waive most of the US sanctions imposed on Iran, which could cripple the nuclear pact.

Responding to criticism that sanctions would be lifted too quickly, Lew said it would not prevent the United States from imposing additional sanctions over issues such as human rights violations if deemed necessary.

Moniz, seeking to counter criticism of ‘loopholes’ in the nuclear inspections regime, said: ‘I am confident that the technical underpinnings of this deal are solid.’

Seeking to reassure Israel and its US supporters, Kerry said Washington would increase security coordination. Prime Minister Benjamin Netanyahu has expressed concerns that Iran will use unfrozen assets to increase funding and weapons to militant groups such as Hamas and Hezbollah.

Kerry said the Iran deal carried the ‘real potential’ for change in the Middle East but acknowledged it ‘does not end the possibility of a confrontation with Iran.’