World powers and Iran launched a new round of negotiations in Istanbul on Saturday, aiming to resolve a long-standing dispute over Tehran's nuclear program that threatens to spark a new war in the Middle East.

"Delegates have gone in... plenary is just getting started," a diplomat close to the negotiations said.

Diplomats say the round, the first in 15 months, is unlikely to result in a major breakthrough but offers a chance to resume dialogue and dampen speculation that Israel might launch military strikes to prevent its arch enemy from acquiring nuclear arms.

Global fuel prices have risen this year amid deepening tensions over the nuclear program, which Iran says is for peaceful purposes.

The meeting is widely seen as a chance for the countries involved - the United States, France, Russia, China, Britain and Germany - and Iran to halt a downward diplomatic spiral and to seek ways out of years of deadlock.

Western diplomats have expressed cautious optimism that Iran, which is OPEC's second-biggest oil producer but has seen its economically vital exports squeezed by increasingly tough sanctions, may finally be ready to discuss curbs to its nuclear program to relieve the pressure.
White House National Security Council spokesman Ben Rhodes said Friday that the Obama administration sees the talks to be held tomorrow as "a positive sign."

"I think nobody expects to resolve all differences in one meeting, but what we want is a positive environment where the Iranian government demonstrates its seriousness and its commitment to pursuing serious negotiations with the P5-plus-1," Rhodes said.

But Iran's English-language state TV station, Press TV, cited sources close to Iran's delegation as saying Tehran saw "few encouraging points" in remarks from U.S. and European officials.

The West accuses Iran of seeking to develop a nuclear weapons capability and Israel - believed to be the only Middle East state with an atomic arsenal - has hinted at pre-emptive military strikes to prevent its foe from obtaining such arms.