America's top general yesterday gave his clearest public warning yet against a "destabilising" Israeli strike on Iran as a senior US security official arrived in Jerusalem amid expectations that he would urge Benjamin Netanyahu to give sanctions time to bite.

The Israeli Prime Minister met Tom Donilon, Washington's National Security Adviser, after General Martin Dempsey, the chairman of the US joint chiefs of staff, declared that it was "not prudent at this point" for Israel to launch an attack on Iran's nuclear facilities, which would not "achieve their long-term objectives".

The US-led calls on Israel to stay its hand came on a day when Tehran said it was halting oil exports to Britain and France in an apparently pre-emptive retaliation against the EU's decision to boycott Iranian oil from July. The terse announcement followed earlier contradictory signs over whether the regime would halt sales to up to six countries in the EU, which accounts for 18 per cent of Iranian oil exports.

The Mehr news agency later said crude exports to Britain and France had been halted and the National Iranian Oil Company had sent a written ultimatum to some European refineries, demanding that they sign long-term agreements of up to five years or be cut off altogether. No details were given of which countries had been threatened, but Spain, Italy and Greece are among its biggest European customers.

Meanwhile in an interview with CNN, General Dempsey said Israel recognised US "concerns" about unilateral military action against Iran, but "I wouldn't suggest, sitting here today, that we've persuaded them that our view is the correct view".

He said sanctions and diplomacy were both "having an effect" and that a military strike would be "premature". Asked about the mindset of Iran's leadership, he said: "We are of the opinion that the Iranian regime is a rational actor. And it's for that reason ... that we think the current path we're on is the most prudent path at this point."

His remarks were echoed by William Hague, Britain's Foreign Secretary, who repeated that a nuclear-armed Iran would result in another Cold War in the Middle East. But he said sanctions and negotiations needed to be given a "real chance" to convince Tehran not to pursue a military nuclear programme. He added: "I don't think a wise thing at this moment is for Israel to launch a military attack on Iran."

Mr Netanyahu is expected to hold talks next month with US President Barack Obama when he travels to Washington for the annual conference of the powerful American Israel Public Affairs Committee.

Iran's foreign minister, Al Akbar Salehi, said yesterday that the next round of negotiations between Tehran and six world powers would be held in Istanbul. He gave no date for the talks and was adamant that Iran would continue with its nuclear programme, which it insists is for peaceful purposes. He said the country remained prepared "even in the worst case scenario" of an external attack.