Several of the world's nuclear powers over the month ahead will discuss progress on a landmark treaty on disarmament, with Israel attending for the first time in two decades as an observer.

Diplomats said the ongoing Iran discussions are separate from the Nuclear Nonproliferation Treaty, or NPT, review conference, which is tasked with setting a path to the elimination of the estimated 16,000 nuclear weapons in the world today.

Immediate concerns at the conference include the lack of progress in disarmament by the United States and Russia, who between them hold more than 90 percent of those weapons. Civil society groups say nuclear powers are spending billions of dollars to instead modernize their arsenals. The United States says it is "maintaining and servicing" instead.

Few breakthroughs are expected at the conference as diplomats warn of Cold War-style tensions over Ukraine and other issues. "I know as well as anyone that we have a long way to go" on the path to a nuclear-free world, Kerry said, acknowledging that "we know that we can cut back even further."