THE United Nations nuclear watchdog chief is increasing the pressure on Iran over suspicions it has researched how to build an atomic bomb – calling for immediate access to a key military site.

Signalling frustration at the lack of progress, Yukiya Amano told the 35-nation board 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 co-operation with inspectors, the International Atomic Energy Agency (IAEA) "cannot conclude all nuclear material in Iran is in peaceful activities", said Mr Amano, the watchdog's director-general.

His message that Iran must act now was also echoed by the United States and Saudi Arabia. They said 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 a pre-emptive strike against Tehran.

"There is a finite amount of time," United States Secretary of State John Kerry said while in Riyadh.

Iran was upbeat last week after talks with world powers in Kazakhstan about its nuclear work ended with an agreement to meet again. However,Western officials said it had yet to take concrete steps to ease their fears about its atomic ambitions.

The US, China, France, Russia, Britain and Ger-many offered modest relief from economic sanctions in return for Iran scaling back its most sensitive nuclear activity, but made it clear they expected no immediate breakthrough.

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