President Barack Obama said the United States and other nations would begin to give Iran "modest relief" on economic sanctions as long as Iran lives up to its end of an agreement reached on Sunday to start implementing a nuclear deal.

Obama's statement came as six world powers and Iran agreed on a six-month plan that would curb Tehran's most sensitive nuclear activities in exchange for some relief from oil and other economic sanctions while the parties negotiate a broader settlement on the scope of Iran's nuclear program.

Obama said the deal was the "first time in a decade" Iran had agreed to halt progress on its nuclear program and said the agreement will help prevent Iran from building a nuclear weapon.

"With today's agreement, we have made concrete progress," Obama said in a statement, noting work now begins on the broader, long-term agreement.

"I have no illusions about how hard it will be to achieve this objective, but for the sake of our national security and the peace and security of the world, now is the time to give diplomacy a chance to succeed," he said.

But Obama also faces pressure from the US Congress to pass new sanctions on Iran as a type of "insurance policy" to push Tehran to abide by the new deal.

Fifty-nine senators - 16 of them Democrats - of the 100 in the chamber have signed on to a bill that would require further cuts in Iran's oil exports.

Obama said on Sunday he would veto the bill if Congress passes it, but said the United States would be ready to increase its sanctions if Iran fails to comply with the terms of the six-month deal.

"Imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully, and I will veto any legislation enacting new sanctions during the negotiation," Obama said in the statement.