US President Barack Obama has sought to reassure Israeli Prime Minister Benjamin Netanyahu in an attempt to head off any premature Israeli strike on Iran.

Mr Obama said the US would always "have Israel's back" but said there was still time for diplomacy.

Mr Netanyahu said that both Israel and the US stood together on the need to prevent Iran from obtaining nuclear weapons.

The talks at the White House were a show of unity between two men who have had a rocky relationship in the past.

"The bond between our two countries is unbreakable," Mr Obama said.

"The United States will always have Israel's back when it comes to Israel's security."

The two men, sitting side-by-side in the Oval Office, sought to present a united front in the Iranian nuclear standoff after weeks of mounting concern that Israel would pre-emptively strike Iran on its own.

In one of the most consequential meetings of US and Israeli leaders in years, they made no mention of any differences they may have over red lines that could trigger military action to curb an Iranian nuclear programme that Israel sees as a threat to its existence.

"We believe there is still a window that allows for a diplomatic resolution," Mr Obama said.

Mr Netanyahu made clear that Israel would be the "master of its fate" in deciding how to deal with Iran, which has called for the destruction of Israel.

"It must have the ability to defend itself, by itself, against any threat," Mr Netanyahu said, echoing remarks Mr Obama made yesterday in a speech to the pro-Israel lobby AIPAC.

Mr Obama has been urging Israel to allow sanctions more time to work against Iran's nuclear ambitions, while balancing that with assurances of his resolve to do whatever is necessary to keep Iran from becoming a nuclear-armed state.

At the White House meeting, Mr Obama told Mr Netanyahu the US reserved "all options" in dealing with Iran.

The US President has made clear that would include a possible military component.

"We do not want to see a nuclear arms race in the most volatile region in the world," Mr Obama said.

Elsewhere, International Atomic Energy Agency Director General Yukiya Amano has said that his organisation could not be sure that Iran's nuclear programme did not have military aims.

Mr Amano has said that "the agency is unable to provide credible assurance about the absence of undeclared nuclear material and activities in Iran and therefore to conclude that all nuclear material in Iran is in peaceful activities".

Mr Amano said that since late last year, Iran had tripled monthly output of higher-grade enriched uranium.