A United Nations nuclear team arrived in Tehran early Sunday for a mission expected to focus on Iran's alleged attempt to develop nuclear weapons.

The chief of the UN nuclear agency says he has reached a deal with Iran on probing suspected work on nuclear weapons and adds that the agreement will "be signed quite soon."

In a rare show of unity, Iran and the world's big powers hailed their first nuclear meeting in more than a year as a key step toward further negotiations meant to ease international fears that Tehran may weaponize its nuclear program.