The U.S. wants Israel to first obtain permission from Washington for any pre-emptive attack on Iran's nuclear facilities but failed to get a clear response, unnamed sources said according to U.K.'s "The Telegraph".

U.S. Defence Secretary Leon Panetta held a secret meeting with Israeli Prime Minister Benjamin Netanyahu last month and officials briefed on the outcome of the talks said the former delivered an urgent message from U.S. President Barack Obama. Panetta reportedly told Netanyahu that Obama is seeking Israel's guarantee that it will not strike Iran without Washington's approval.

One of the sources, who are from both Israel and the U.S., revealed that Netanyahu and Israel Defence Minister Ehud Barak did not give Panetta any hint of an attack plan nor assurance that they will not act unilaterally and seek Washington's nod first.

Meanwhile, an explosion in an Iranian ammunition depot on Saturday came four days after the International Atomic Energy Agency released a report linking Iran's nuclear program to the building of a nuclear weapon. Iran said it was an accident but a Western intelligence source, according to Time, claims the target was a depot of Shahab missiles and Israel's spy agency is behind the sabotage. The long-range missile can reach Israel.

The explosion in the village of Bidganeh, near the city of Karaj reportedly killed 16 soldiers and missile expert Maj. Gen. Hassan Moqqadam. The blast was heard in Tehran, 40 kilometres away in the east, shattered the windows of nearby buildings and caused a fire at the base.

It was not the first time that an arms depot or a missile facility was hit by an explosion. In October 2010 and November 2007, similar facilities were hit by explosions. Last year, two Iranian scientists working in the nuclear program were assassinated and another one was killed in July.

Computers used in Iran's nuclear centrifuges were also downed by the so-called Stuxnet virus for months disrupting Iran's uranium enrichment process.

For its part, Israel recently tested a ballistic missile that can reach Iran. Previously, Israel's air force conducted a war exercise with NATO in Sardinia and a civilian air raid drill in Tel Aviv was conducted.

Iran continues to deny that its nuclear program is for military purpose and insists that it is for energy self-sufficiency. But Western countries led by the U.S. and Israel doubt this and have imposed economic sanctions on Iran as a punishment.