Washington and London sought to ease concerns over the nuclear accord with Iran, even as the Islamic state's supreme leader warned 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 the deal with Israel's arch-foe as a "historic mistake" and hinted at a possible military response.
The 10-year agreement struck in Vienna this week calls for a lifting of the sanctions that have crippled the Iranian economy in exchange for measures to ensure Iran does not build nuclear weapons.
Hammond sought to reassure Israel during a visit to Jerusalem, saying "robust measures" were in place to make sure it is successful.
"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 Defense Secretary Ashton Carter is also due in Israel next week and Israeli opposition leader Isaac Herzog is to visit Washington, partly to push for security guarantees.
US Secretary of State John Kerry will also head to the Gulf next month to allay 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.
"If Iran should try to cause mischief in the region, we're committed to confront it resolutely," said Jubeir.
- UN backing -
The UN Security Council is now expected to endorse the deal on Monday, diplomats said.
The resolution is expected to pass without difficulty as its five veto-wielding permanent members -- Britain, China, France, Russia and the United States -- negotiated the accord, along with Germany.
The new resolution would replace the existing framework of seven sets of Security Council sanctions imposed on Iran since 2006, enshrining a new set of restrictions.
But a UN embargo on conventional arms sales and exports is to stay in place for five years, while trade in ballistic missiles capable of being armed with a nuclear warhead will remain for eight years.
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US President Barack Obama on Wednesday said opponents to the deal 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.
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 that Washington's alliances in the Middle East would not be affected.
- 'Not trustworthy' -
In a boost to the agreement, more than 100 former US ambassadors, including the ranking diplomat taken hostage in Tehran in 1979 when students stormed the American embassy, praised it as a "landmark".
"If properly implemented," they said in an open letter, it "can be an effective instrument in arresting Iran's nuclear program".
But in Iran, 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 added, without specifying which.
Khamenei often spoke of his distrust of Washington during almost two years of negotiations that culminated in the Vienna agreement after a final 18-day stretch of almost round-the-clock talks.
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.