These are the shock revelations released in a report according to the Los Angeles Times which published details about Israel's new capability on Saturday.
The missile modification was described by two US officials and confirmed by an Israeli official, the Times said. All three spoke on condition of anonymity.
"The Americans said they were disclosing the information to caution Israel's enemies at a time of heightened tensions in the region and concern over Iran's alleged ambitions," the Times said.
"We tolerate nuclear weapons in Israel for the same reason we tolerate them in Britain and France," a senior US official was quoted as saying. "We don't regard Israel as a threat."
Israel's newly acquired submarine capability will complicate efforts to persuade Iran to abandon a suspected nuclear weapons programme, the Times said.
Arab countries have criticised the United States and United Nations for pressuring Iran to accept tougher inspections while ignoring Israel's stockpile.
"A big source of contention is Israel," said a senior UN official trying to convince Iran to undergo additional inspections. "This is a magnet for other countries to develop nuclear weapons."