Obama: If Iran cheats, the world will know

US President Barack Obama has welcomed a tentative deal on Iran's nuclear program as "historic," but also warned Tehran that the world was watching to be sure it fulfilled its commitments.

"If Iran cheats, the world will know it," Obama said on Thursday, saying the deal reached after marathon talks in Switzerland was based not on trust but on "unprecedented verification."

Meanwhile, Obama says he will speak to Prime Minister Benjamin Netanyahu to reassure him of steadfast US support for Israel's security despite the landmark nuclear deal.

Netanyahu had bitterly opposed an agreement between the world powers and Iran. But Obama said he would call the prime minister "to make clear there will be no daylight."

"There is no daylight when it comes to our support for Israel's security and our concerns about Iran's destabilising policies and threats towards Israel," Obama said.