TEL AVIV (AP) - United States Defence Secretary Chuck Hagel said on Sunday the United States and Israel see "exactly the same" threat from Iran, but differ on when it may reach the point of requiring US or Israeli military action.

Mr Hagel used his first visit to Israel as Pentagon chief to highlight his view that Israel must decide for itself whether and when to pre-emptively attack its neighbour.

"Israel will make the decision that Israel must make to protect itself, to defend itself," Mr Hagel told reporters before arriving here on Sunday to begin a weeklong tour of the Middle East.

Mr Hagel acknowledged that while Israel and the US share a commitment to ensuring that Iran does not acquire a nuclear weapon, there "may well be some differences" between the two allies on the question of when Iran's leaders might decide to go for a bomb.

He said there is "no daylight at all" between Israel and the US on the central goal of preventing a nuclear-armed Iran.

However, he added: "When you back down into the specifics of the timing of when and if Iran decides to pursue a nuclear weapon, there may well be some differences."

Israeli Prime Minister Benjamin Netanyahu tends to see more urgency, reflecting in part the fact that certain Iranian technological advances toward a nuclear weapon could put the programme beyond the ability of the Israeli military to destroy it with airstrikes.

The first thing Hagel did upon arrival in Jerusalem was take a guided tour of the Yad Vashem Holocaust history museum, participate in a ceremony at the Hall of Remembrance and write an inscription in the guest book at a memorial for the 1.5 million Jewish children who perished in the Holocaust.

"There is no more poignant, more touching, more effective way to tell the story than this reality, as painful as it is, but it is a reality," he said after completing his visit. "It did happen, and we must prepare our future generations ... for a clear understanding that we must never allow this to happen again."