By Arshad Mohammed and Angus McDowall
RIYADH (Reuters) - Secretary of State John Kerry said on Monday there was "finite" time for talks between Iran and world powers on its disputed nuclear program to bear fruit, but gave no hint how long Washington may be willing to negotiate.
Israel, Iran's arch-enemy and convinced Tehran is secretly trying to develop nuclear weapons, has grown impatient with the protracted talks and has threatened pre-emptive war against Tehran if it deems diplomacy ultimately futile.
Kerry's sentiment was largely echoed by Saudi Foreign Minister Prince Saud al-Faisal, who said that the negotiations cannot be endless like the debates of philosophers over how many angels can fit on the head of a pin.
"There is a finite amount of time," Kerry, in the Saudi capital Riyadh on his first overseas trip as the top U.S. diplomat, said of the talks between a group of six world powers and Tehran, Saudi Arabia's main regional adversary.
Kerry was speaking at a news conference with Prince Saud al-Faisal, who suggested Iran was not showing enough seriousness about the discussions, which he said "cannot go on forever".
Iran was positive last week after talks with the powers in Kazakhstan about its nuclear work ended with an agreement to meet again. But Western officials said it had yet to do anything concrete to allay their concerns about its nuclear aspirations.
The United States, China, France, Russia, Britain and Germany offered modest relief from economic sanctions in return for Iran reining in its most sensitive nuclear activity but made clear that no breakthrough was in the offing quickly.
"We can't be like the philosophers who keep talking about how many angels a pinhead can hold," Prince Saud al-Faisal said.
"They (the Iranians) have not proved to anybody the urgency in their negotiation," he said. "They reach common understanding only on issues that require further negotiation. And so this is what (has) worried us."
The United States and many of its allies suspect Iran may be using its civil nuclear program as a cover to develop atomic weapons, a possibility that Israel, which is regarded as the Middle East's only nuclear power, sees as a mortal threat.
The possibility also deeply disturbs many Arab countries in the Gulf who, some analysts say, could choose to pursue their own nuclear programs if Iran were to acquire an atomic bomb, leading to a destabilizing arms race.
In Vienna on Monday, the U.N. nuclear watchdog raised pressure on Iran to finally address suspicions that it has sought to design an atomic bomb, calling for swift inspector access to a military base where relevant explosives tests are believed to have been carried out.
DIPLOMACY "FIRST CHOICE"
Iran says its program is solely for peaceful purposes, such as generating electricity and making medical isotopes.
Kerry, in the final stages of a nine-nation, 11-day trip that will also take him to Abu Dhabi and Doha, also had lunch with Palestinian President Mahmoud Abbas to discuss the possibility of reviving peace talks with Israel.
Making his first trip abroad as secretary of state, Kerry also met Saudi Crown Prince Salman but a U.S. official said he would not see Saudi King Abdullah, who turns 90 this year.
Kerry said a diplomatic solution on Iran is still preferred by the United States and Saudi Arabia.
In 2008, Riyadh's ambassador to Washington said King Abdullah had repeatedly urged Washington to "cut off the head of the snake" by striking Iran's nuclear facilities, according to a U.S. diplomatic cable released by WikiLeaks.
"We both prefer - and this is important for Iranians to hear and understand - we both prefer diplomacy as the first choice, the preferred choice," Kerry said. "But the window for a diplomatic solution simply cannot by definition remain open indefinitely."
Echoing Western concerns about a possible nuclear arms race in the Middle East in the event that Iran obtained a nuclear bomb, Kerry made a series of arguments for Gulf Arab countries not to pursue a military nuclear capability.
These included standing U.S. policy to prevent Iran from acquiring such arms, the dangers of nuclear proliferation, the diversion of resources that could otherwise go to economic development, and the general trend by the United States and Russia toward reducing their doomsday arsenals.
"The threat is not just the threat of a nuclear bomb, the threat is also the threat of a dirty bomb or of nuclear material being used by terrorists," said Kerry.
In December 2011, former Saudi intelligence chief Prince Turki al-Faisal said that if Tehran did gain nuclear weapons capability, Saudi Arabia should consider matching it.
Riyadh has also announced plans to develop 17 gigawatts of atomic energy by 2032 as it moves to reduce domestic oil consumption, freeing up more crude for export.
(For an interactive timeline on Iran's nuclear program, click on http://link.reuters.com/gad76r)
(Reporting by Arshad Mohammed, Angus McDowall and Mahmoud Habboush; writing by Sami Aboudi; editing by William Maclean and Mark Heinrich)