US will 'never allow' Iran to have a nuclear weapon: Pence

"I have a solemn promise to Israel, to all the Middle East and to the world - the United States of America will never allow Iran to acquire a nuclear weapon," Pence said to applause from Israeli MPs.

The 2015 deal that was meant to curb Iran's nuclear capabilities was fiercely opposed by Israel, but backed by then US president Barack Obama.

His successor Donald Trump has fiercely criticised the agreement, accusing Tehran of not sticking to it and saying it still allows the Islamic republic to support terrorist organisations across the globe - including Israel's longstanding enemies.

Earlier this month Trump again waived nuclear-related sanctions - as required every few months to stay in the agreement - but demanded European partners work with Washington to improve the terms of the deal.

"The Iran nuclear deal is a disaster and the United States of America will no longer certify this ill-conceived agreement," Pence said today.

"Unless the Iran nuclear deal is fixed President Trump has said the United States will withdraw from the Iran nuclear deal immediately."

The other parties to the deal - Britain, China, France, Germany, Russia and the European Union - have all said it is working and that Iran is complying fully with its commitments.