Washington and London sought Thursday to ease concerns over the Iran nuclear accord as the country's supreme leader warned that major powers are not to be trusted over its implementation.

As part of an international charm offensive, British Foreign Secretary Philip Hammond met Israeli Prime Minister Benjamin Netanyahu, who strongly opposes the landmark accord struck on Tuesday in Vienna between Iran and six powers including Washington.

Netanyahu has condemned it as a "historic mistake" and hinted at a possible military response.

Hammond sought to reassure Israel during a visit to Jerusalem, saying "robust measures" were in place to ensure the success of the nuclear deal with its arch-foe Iran.

"Our focus now will be on swift and full implementation of the agreement to make sure that a nuclear weapon remains beyond Iran's reach," he said.

US Defence Secretary Ashton Carter is also due in Israel next week while Israeli opposition leader Isaac Herzog is to visit Washington, partly to push for security guarantees.

US Secretary of State John Kerry will head to the Gulf next month to ally fears over the nuclear deal.

The announcement came after Kerry held talks in Washington with Saudi Foreign Minister Adel al-Jubeir, who used the occasion to warn Iran against any further "adventures" in the region.

- Saudi to 'confront mischief' -

"If Iran should try to cause mischief in the region, we're committed to confront it resolutely," said Jubeir.

As the freshly inked deal was put to members of the UN Security Council, US President Barack Obama said opponents in the United States and abroad had offered only a path to war.

"If 99 percent of the world community and the majority of nuclear experts look at this thing and they say this will prevent Iran from getting a nuclear bomb, and you are arguing either that it does not or that even if it does, it's temporary... then you should have some alternative," Obama said.

The issue is either resolved "diplomatically, through a negotiation, or it's resolved through force. Through war. Those are the options."

Obama's Republican rivals, who hope to scupper the agreement in a planned Congressional vote, have accused him of appeasement.

The president has said he will veto any attempt to block the deal.

The agreement, struck after two years of tough negotiations, aims to roll back Iran's nuclear programme in return for lifting sanctions that have crippled its economy.

Obama also addressed the concerns of Saudi Arabia and other Sunni Gulf states that the accord legitimises what they see as Iranian interference in the oil-rich region.

The agreement would not end "profound differences" with the Shiite-majority Islamic republic, he said, stressing their alliances with Washington would remain unchanged.

- 'Not trustworthy' -

In a letter to the United Nations, the six powers said Iran would remain under the threat of renewed sanctions for 15 years if it does not live up to its commitments.

While Obama defended the deal in Washington, Iran's supreme leader Ayatollah Ali Khamenei warned President Hassan Rouhani that "some" world powers are not to be trusted in implementing it.

In a letter to Rouhani, Khamenei, who had the final word on the agreement, congratulated Iran's negotiators for "their tireless efforts".

Bringing the talks to a close was a "milestone", Khamenei said, but the agreement requires "careful scrutiny" before it is approved.

Rouhani "must be concerned about possible violation of commitments by the other parties and close paths to it," the leader wrote in the letter, published on his website.

"You are well aware that some of the six states participating in negotiations are not trustworthy at all," Khamenei said.

He did not specify which of the six countries -- Britain, China, France, Russia and the United States plus Germany -- he was referring to.

Throughout the almost two years of negotiations that culminated in the Vienna agreement after a final 18-day stretch of almost round-the-clock talks, Khamenei often spoke of his distrust of Washington.

Iran has always denied Israeli and Western charges of seeking an atomic weapons capability, insisting its nuclear programme is for peaceful energy and medical purposes only.

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