US President Barack Obama has earned sufficient congressional backing to ensure the international deal to roll back Iran's nuclear program goes into effect, with a 34th senator announcing her support for the accord.

Most US lawmakers are opposed to the deal, which would ease punishing economic sanctions on Tehran while preventing Iran from advancing its nuclear program.

Many Republicans warn that Iran will seek to cheat its way to an atomic bomb.

Should Congress pass a resolution that disapproves of the deal, Obama would veto it.

Overcoming the veto would require a two-thirds vote in both the Senate and the House of Representatives.

With Democrat Barbara Mikulski's support, the deal now has 34 backers in the Senate - the number needed to uphold Obama's veto.

Mikulski said on Wednesday that while the deal is not perfect, "I have concluded that this Joint Comprehensive Plan of Action is the best option available to block Iran from having a nuclear bomb".

She added that while she will vote for the deal, "Congress must also reaffirm our commitment to the safety and security of Israel."

Republicans are unified in their opposition to the Iran accord.

Two top Democratic senators - Chuck Schumer and Robert Menendez - are also opposed, as are a handful of Democrats in the House, including Steve Israel, the chamber's highest-ranking Jewish member.

Congress is set to vote on the accord later this month.