VIENNA - Six world powers demanded on Thursday that Iran fulfil a pledge to let international inspectors visit a military installation where the UN nuclear watchdog says explosives research geared to developing atomic bombs may have taken place.

The joint call demonstrated unusual unity among the powers on Iran ahead of a planned revival of high-level talks as well as widening disquiet about the nature of Tehran’s nuclear quest, with Israel threatening last-ditch military action.

Iranian Supreme Leader Ayatollah Ali Khamenei welcomed comments by US President Obama about a diplomatic “window of opportunity” offered by renewed talks, but said Washington’s simultaneous moves to “bring the Iranian people to their knees” with harsh sanctions were driven by delusion.

Heaping pressure on Iran to come clean on its nuclear activity, the United States, Russia, China, Britain, France and Germany used a UN nuclear watchdog governors’ meeting to urge Tehran to grant prompt access to its Parchin military facility.

They voiced concerned that no deal was reached between Iran and International Atomic Energy Agency inspectors at talks in January and February, “including on the access to relevant sites in Iran, requested by the agency ... In that context we urge Iran to fulfil its undertaking to grant access to Parchin.”

Iran has said inspectors can go to Parchin, but only after a broader deal is reached on how to address all outstanding issues between Tehran and the UN agency — an approach Western diplomats dismissed as a procedural stalling tactic.

Western diplomats said this week they suspect Iran might be delaying a UN inspectors’ trip to Parchin while it clears away evidence of explosives tests relevant to designing atomic bombs.

The six powers made no such accusation in their statement at a closed-door board session of the 35-nation International Atomic Energy Agency governing board.

According to AFP the United States offered Israel advanced weaponry in return for it committing not to attack Iran’s nuclear facilities this year, Israeli daily Maariv reported on Thursday. Citing unnamed Western diplomats and intelligence sources, the report said that during Prime Minister Benjamin Netanyahu’s visit to Washington this week, the US administration offered to supply Israel with advanced bunker-busting bombs and long-range refuelling planes. In return, Israel would agree to put off a possible attack on Iran till 2013, after the US elections in November.

Israel and much of the international community fear Iran’s nuclear programme masks a weapons drive, a charge Tehran denies, and it was top of the agenda at talks between Netanyahu and US President Barack Obama in Washington this week.

The United States and Israel are at odds over just how immediate the Iranian threat is. Netanyahu said on Monday that sanctions against Iran have not worked, and “none of us can afford to wait much longer.”

A key difference between Washington and Israel has emerged on the timeline available for a military strike against Iran, with the Jewish state warning that the weaponry available to it gives it a shorter window for action.

In response, the report said, the US administration offered to give Israel weapons and material that could extend its window to act against Iran.

In particular, it would offer bunker-busting bombs more powerful than those currently possessed by Israel, which would allow the Jewish state to target Iranian facilities even under solid rock.

Earlier this week, the Ynet website reported said that Israel Military Industries (IMI), one of the country’s leading weapons manufacturers, had upgraded its MPR-500 guided missiles, turning them into “bunker busters” capable of penetrating double-reinforced concrete walls and floors 200mm thick.

But figures from a poll published on Thursday showed that almost six out of 10 Israelis — or 58 percent — were against the idea of a pre-emptive strike on Iran’s nuclear facilities without US backing.

The same poll, published by Haaretz newspaper, also found that over half of the respondents trusted Netanyahu and Defence Minister Ehud Barak to handle the Iran issue.

The Maariv report comes shortly after world powers known as the P5+1 — five UN Security Council members plus Germany — offered to resume long-stalled talks with Tehran over its contested nuclear programme.

Israel has cautiously welcomed the talks, but warned it must be prepared for the potential failure of any new dialogue with Iran.