A US newspaper reported in June that Israel had rehearsed an attack on Iran
US President George W Bush told Israel's prime minister in May that the US would not back an attack on Iran, a UK newspaper has reported.
A spokesman denied Ehud Olmert had said the words attributed to him in a working meeting with a foreign guest.
The Guardian quoted unnamed European diplomatic sources as saying Mr Olmert had used a one-to-one meeting with Mr Bush in May to raise the issue.
Israel sees Iran's nuclear programme as its greatest threat.
Iran insists its nuclear programme is for peaceful purposes only but has defied demands from the United Nations Security Council that it halt enriching uranium.
According to the sources quoted by the Guardian, Mr Bush turned down the proposal for an attack and said the US position was unlikely to change as long as he was in office.
Israeli government spokesman Mark Regev denied the report when the Guardian put the claims to him:
"The need to prevent Iran from obtaining nuclear weapons is raised at every meeting between the prime minister and foreign leaders. Israel prefers a diplomatic solution to this issue but all options must remain on the table.
"Your unnamed European source attributed words to the prime minister that were not spoken in any working meeting with foreign guests."
A US National Security Council spokesman, Gordon Johndroe, said that diplomacy remained America's first course of action while all options remained on the table.
Speaking at the UN General Assembly on Wednesday, Israeli President Shimon Peres said Iran was "at the centre of violence and fanaticism" in the world.
He warned that the Tehran leadership was continuing to enrich uranium and develop long-range missiles.
In June this year, the New York Times newspaper reported that Israel had carried out an apparent rehearsal for an attack on Iran's nuclear facilities.
More than 100 Israeli fighter jets took part in manoeuvres over the eastern Mediterranean and over Greece in the first week of June, US officials quoted by the newspaper said.