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.