VIENNA: 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.

Airing frustration at the lack of progress in his agency's investigation, Yukiya Amano told its 35-nation governing board on Monday that negotiations with Iran must "proceed with a sense of urgency" and be focused on achieving concrete results soon.

Because Iran is not providing the necessary cooperation with inspectors, the International Atomic Energy Agency "cannot conclude that all nuclear material in Iran is in peaceful activities", said Amano, the IAEA's director-general.

His message that Iran must act now was echoed by the United States and its top Gulf ally Saudi Arabia. They declared on Monday that separate but related talks between Tehran and world powers on a wider diplomatic solution to the nuclear dispute could not go on indefinitely.

Israel, Iran's arch-enemy and convinced Tehran is secretly trying to develop a nuclear weapon, has grown impatient with the protracted talks and has threatened pre-emptive war against Tehran if it deems diplomacy ultimately futile.

"There is a finite amount of time," U.S. Secretary of State John Kerry, in Riyadh, said of the talks between a group of six world powers and Tehran, Saudi Arabia's main regional adversary.

In Washington, Vice President Joe Biden told America's biggest pro-Israel lobbying group in a speech that President Barack Obama is "not bluffing" about U.S. determination to stop Iran from getting a nuclear weapon.

"We're not looking for war. We're ready to negotiate peacefully. But all options including military force are on the table. While that window is closing, we believe there is still time and space (for diplomacy)," Biden said.

But Israeli Prime Minister Benjamin Netanyahu, addressing the same gathering, said neither diplomacy nor sanctions had stopped Iran from pursuing its nuclear ambitions and a clear and credible military threat was now needed.

Iran was upbeat 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 take concrete steps to ease their fears about its atomic ambitions.

The United States, China, France, Russia, Britain and Germany offered modest relief from economic sanctions in return for Iran scaling back its most sensitive nuclear activity, but made clear that they expected no immediate breakthrough.

The IAEA has been trying separately for more than a year to persuade Iran to cooperate with a long-stalled agency investigation into suspected nuclear weapons research by Tehran, which denies any such activity.

The U.N. agency's priority is to be able to inspect Parchin, a sprawling military site southeast of the capital Tehran, where it believes Iran built an explosives chamber to carry out tests, possibly a decade ago. Iran denies this.

The IAEA has been asking for Parchin access for over a year.

Iran says it first needs to agree with the IAEA on how the inquiry is to be conducted before allowing any Parchin visit. But Amano underlined that access should be granted in any case, even before a deal on investigation ground rules was reached.

He told the IAEA governors that he was "once again unable to report any progress on the clarification of outstanding issues, including those relating to the possible military dimensions to Iran's nuclear programme".

Some diplomats and analysts say Iran is using the meetings with the IAEA merely for leverage in its negotiations with world powers which, unlike the IAEA, have the power to ease sanctions that they have recently tightened on the major oil producer.

"Providing access to the Parchin site would be a positive step which would help to demonstrate Iran's willingness to engage with the agency on the substance of our concerns," Amano said, according to a copy if his speech.

Western officials accuse Iran of cleansing the Parchin site of any incriminating evidence of illicit nuclear-related activity, a charge the Islamic Republic has dismissed.

Citing satellite imagery, they say Iran now seems to be rebuilding the specific part of Parchin that inspectors want to see, after last year razing several smaller buildings there.

Amano also said Iran was continuing to construct a research reactor at Arak, which Western experts say could offer the Islamic state a second way of producing material for a nuclear bomb, if it decided to embark on such a course.

"Iran has stated that the reactor is expected to begin operating in the first quarter of 2014," Amano said.

Western worries about Iran are focused largely on uranium enrichment plants at Natanz and Fordow, as such material refined to a high level can provide the fissile core of an atomic bomb.

But experts say Arak could yield plutonium for bombs if the spent fuel is reprocessed, something Iran has said it has no intention of doing.

Iran, a leading oil producer, says its nuclear programme is entirely peaceful and aimed primarily at producing electricity.

But its refusal to curb atomic activity which can have both civilian and military purposes and its lack of full openness with U.N. inspectors have drawn increasingly tough Western sanctions targeting its lifeblood oil exports.