US President Barack Obama launched an aggressive and detailed defense of a landmark Iranian nuclear accord, rejecting the idea that it leaves Teheran on the brink of a bomb and arguing the only alternative to the diplomatic deal is war.

The president vigorously challenged his critics during a lengthy White House news conference on Wednesday, a day after Iran, the US and five other world powers finalized a historic yearslong agreement to curb Teheran's nuclear program in exchange for billions of dollars in sanctions relief.

Opposition to the deal has been fierce, both in Washington and Israel. Sunni Arab rivals of Shiite Iran also expressed concerns.

"Either the issue of Iran obtaining a nuclear weapon is resolved diplomatically through negotiation, or it's resolved through force, through war," Obama said. "Those are the options."

"If 99 percent of the world community and the majority of nuclear experts look at this thing and they say this will prevent Iran from getting a nuclear bomb, and you are arguing either that it does not or that even if it does, it's temporary ... then you should have some alternative." Obama said.

Israeli Prime Minister Benjamin Netanyahu, perhaps the fiercest critic of Obama's overtures to Iran, showed no sign he could be persuaded to even tolerate the agreement. In remarks to Israel's Parliament, Netanyahu said he was not bound by the terms of the deal and could still take military action against Iran. He sees Iran's suspected pursuit of a nuclear weapon as a threat to Israel's existence.

In the US Congress, resistance comes not only from Republicans, but also the Democratic Party. Vice-President Joe Biden spent the morning on Capitol Hill meeting privately with House Democrats, and planned to return on Thursday to make a similar pitch to Democrats on the Senate Foreign Relations Committee.

AP - AFP