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U.S. Secretary of State John Kerry flew from London to Geneva on Sunday to meet British and Libyan officials after taking part in breakthrough talks curbing Iran's nuclear activity.

Iran and six world powers clinched a deal on Sunday curbing the Iranian nuclear program in exchange for initial sanctions relief, signaling the start of a game-changing rapprochement that could ease the risk of a wider Middle East war.

Aimed at ending a long festering standoff, the interim pact between Iran and the United States, France, Germany, Britain, China and Russia won the critical endorsement of Iranian clerical Supreme Leader Ayatollah Ali Khamenei.

The United States and some of its allies suspect Iran of using its civilian nuclear program as a cover to develop nuclear weapons. Tehran denies this, saying its program is for purely peaceful purposes such as generating electricity.