General Martin Dempsey, at the start of a trip to Afghanistan and Iraq, said late Sunday that Israel viewed the threat posed by Tehran's atomic ambitions with more urgency, as a nuclear-armed Iran could endanger Israel's very existence.

Dempsey, the chairman of the US Joint Chiefs of Staff, said he conferred with his Israeli counterpart Benny Gantz on a regular, "bi-weekly" basis.

"We compare intelligence, we discuss regional implications. And we've admitted to each other that our clocks are turning at different rates," he said.

"They are living with an existential concern that we are not living with."

Amid intense speculation in the Israeli press that Israel soon may launch a unilateral strike against Iran's nuclear sites, Dempsey said the US military felt no pressure from Israel to back possible bombing raids.

Speaking to reporters aboard his plane before landing Sunday night at Bagram air base in Afghanistan, Dempsey reiterated his view that any air strikes by Israel would delay but not destroy Iran's disputed nuclear project.

Similar remarks by the general last week were widely analysed in the Israeli media but Dempsey said his comments were not designed with an Israeli audience in mind.

"You can take two countries and interpret the same intelligence and come out with two different conclusions. I'd suggest to you that's what's really happening here," he said.

The four-star general also struck a cautious note on any possible US military action against the Syrian regime.

Asked about the possibility of the United States backing a no-fly zone over Syria, Dempsey hinted that such a move might draw away military resources needed to deter Iran in the Gulf.

"I am helping shape the discussion (on potential military options for Syria)," he said.

"And I'll tell you we have to clearly define our national interest. What you do in one place, whether it's Syria or the Strait of Hormuz, there are trade offs.

"It's a conversation that we should proceed with cautiously."

Yaakov Amidror, head of the National Security Council, visited Yosef, whose party is a member of Netanyahu's ruling coalition, at his home in Jerusalem on Friday to discuss the issue, Israeli media reports said.

A senior Israeli official, speaking on condition of anonymity, confirmed news of the visit to AFP, but did not give details on the discussions.

Thus far, Yosef has ordered Shas ministers to oppose any Israeli attack on Iran's nuclear facilities that is not coordinated with the United States.

Israeli commentators have said that Netanyahu and Defence Minister Ehud Barak do not have a majority in the country's cabinet or the security cabinet to carry out an offensive against Iran, hence the importance of Shas.

Army radio reported that, since the visit, Yosef has become less determined in his opposition to an Israeli attack.

It cited extracts from a religious commentary made on Saturday night in which Yosef likened Iranian leaders to Amman, a Persian enemy of Israel in Biblical times.

"We are all in danger," Yosef was quoted as saying. "We cannot rely on anyone except our father who is in heaven."

Israel believes the Islamic republic is trying to develop a military nuclear capability under the guise of its civilian programme which it says would pose an existential threat to the Jewish state.

Iran denies these accusations.