Fearing a softening of America's stance towards Iran, Israel has sent its highest ranking defence officials to Washington to stress the need for leaving all options open when dealing with Tehran's nuclear threat.

Israel's defence minister, its chief of staff and two other ministers have held talks in Washington over the past fortnight to persuade the US not to abandon a twin approach toward Iran: increasing economic and diplomatic pressure while making clear that the use of military force will not be ruled out.

Israel's concern was raised following a series of public remarks by top US defence officials who made clear America was reluctant to take on a military operation against Iran. Admiral Michael Mullen, chairman of the Joint Chiefs of Staff, recently said that opening another front - in addition to the wars going on in Iraq and Afghanistan - would be "extremely stressing" for US forces. Defence Secretary Robert Gates also made clear his opposition to embarking on a military operation in Iran.

Israel's message has been subtle, to ensure it is not seen as pushing America to war. Officials stressed support for diplomacy as the main avenue to pursue at present, but underlined the need "to prepare for all other options" as chief of staff Gabi Ashkenazi said after meeting with his Pentagon counterparts last week.

"Every day that passes, Iran moves closer to achieving nuclear ability and every day sanctions get closer to stopping Iran getting there. The question is, what will happen first," said an Israeli official.

The administration insists that its policy is unchanged, and both presidential candidates, Republican John McCain and Democrat Barack Obama, support leaving the military option open.

The US is also seeking to ease Israeli concerns by upgrading Israel's early warning systems to protect it from a possible Iranian missile attack. Mr Gates promised Mr Barak that the US would supply Israel with an advance X-band radar system, which can provide information on high altitude long-range missiles.

America, Mr Gates said, would also share real time data from its spy satellites which can detect a missile launch from any point on the globe within seconds.

"These agreements will increase Israel's safety and its ability to defend itself from an attack," Mr Barak said after his meetings in Washington.