Quoting an unnamed American source close to Obama's administration, the Haaretz daily said Washington would pledge under the proposed "nuclear umbrella" to respond to any Iranian nuclear strike against Israel with a U.S. retaliation in kind.

Iran denies its nuclear programme has military designs. But virulent anti-Israel rhetoric from Tehran has spread fears that the Israelis, who are believed to have the Middle East's only atomic arsenal, could attack their arch-foe pre-emptively.

The latitude for unilateral Israeli action might be limited by a U.S. nuclear umbrella. Similar Cold War treaties -- NATO in Europe, the nuclear umbrella over Japan -- defended U.S. allies while obliging them to get Washington's nod for military moves.

Asked about the Haaretz report, an official in Israeli Prime Minister Ehud Olmert's government said only: "We do not engage in speculation whose source is unclear."

An aide to rightist opposition leader Benjamin Netanyahu, who leads a race to replace Olmert in a Feb. 10 election and who has said he believes Obama is serious about preventing Iran from attaining nuclear weapons, declined to comment on the report.

Speculation on the possibility of a U.S.-Israeli strategic pact was stirred two years ago, when President George W. Bush said in an interview with Reuters that his country would "rise to Israel's defence" in the face of Iranian threats.

Obama succeeds Bush in January. A spokesman for the U.S. embassy in Tel Aviv said he could make no statement "on what a future administration's policy might or might not be".

Israel was founded partly as a haven for survivors of the Nazi Holocaust, on the promise that Jews would now look to their own defence. Formally submitting to foreign protection could spell a major credibility crisis for the Israeli government.