London/Washington: Saudi Arabia will adopt a “new defence doctrine” focused on resisting Iranian influence in the Middle East, a senior diplomatic adviser warned on Monday, after the nuclear deal struck with Tehran by six world powers including the US.

As Britain urged the main regional powers to back the agreement, the Saudis offered their grudging support, with an official statement saying that it “could be a first step towards a comprehensive solution for Iran’s nuclear programme, if there are good intentions”.

But the kingdom’s rulers remain deeply suspicious of Iran’s intentions — and almost equally wary of America’s diplomacy, especially since they were kept in the dark about the US contacts with Iran that preceded the Geneva agreement.

Nawaf Obaid, a counsellor to Prince Mohammad Bin Nawaf, the Saudi ambassador to London, accused America of dishonesty. “We were lied to, things were hidden from us,” he said. “The problem is not with the deal struck in Geneva, but how it was done.”

The response, said Obaid, would be a “new defence doctrine” based on containing Iran. Saudi Arabia “will be there to stop them wherever they are in Arab countries”, he said. “We cannot accept Revolutionary Guards running round Homs [in Syria].”

His comments reflect Saudi fears that America’s overtures towards Iran could upset their own close alliance with Washington. Although Saudi Arabia has great wealth, its military strength is limited and its 75,000-strong army is barely a fifth of the size of Iran’s — forcing the country to rely on America as the ultimate guarantor of its territorial security.

“There is no absolute replacement for the US for Saudi Arabia as it casts around for allies, but there is a longer term project of looking to spread its focus,” said Daniel Levy, of the European Council for Foreign Relations. Reflecting official opinion, Arab News, a Saudi newspaper, yesterday carried the main headline: “Nuclear deal sparks Iran hegemony fears.”

Saudi Arabia’s worries are shared by Israel, which also believes that America is naive about the Iranian threat. But William Hague, the British Foreign Secretary, told MPs the agreement ensured that “elements of Iran’s nuclear programme that are thought to present the greatest risk cannot make progress”.

He added: “If Iran implements the deal in good faith as it has undertaken to do, it cannot use these routes to move closer towards obtaining a nuclear weapons capability.” He urged other countries to support the deal and warned against the consequences of undermining it.

Opposition to it remains a potential threat in the US, despite President Barack Obama’s endorsement. In Washington, leading senators are preparing legislation to restore the sanctions eased under the Geneva deal, if Iran reneges.

Meanwhile the final text of the accord called into question a statement by John Kerry, the US secretary of state, after the talks concluded, when he said: “The first step does not say that Iran has a right to enrichment.”

The document says that under the final settlement being aimed at, Iran would have a “mutually defined enrichment programme”. For now, Iran may continue enriching uranium with its 10,000 operational centrifuges, providing it does not exceed the five per cent level needed for nuclear power stations, or add to the overall stockpile.

This suggests Mohammad Javad Zarif, the Iranian foreign minister, was right to say: “This recognition is there — that Iran will have an enrichment programme.”

Hawks in the US Congress are also suspicious of the secret diplomacy that paved the way for the agreement. Oman hosted covert meetings between Iranian and US diplomats and Kerry himself visited Oman in December 2011, one year before becoming secretary of state.

“There is very little in trust in Congress that the president will hold Iran’s feet to the fire,” said a senior Republican Senate aide.

Obama defended the deal, insisting “tough talk” alone would not guarantee US security. “Huge challenges remain, but we cannot close the door on diplomacy, and we cannot rule out peaceful solutions to the world’s problems,” he said.

Benjamin Netanyahu, the Israeli prime minister, said his influence during the two rounds of talks had prevented an even worse deal.