WASHINGTON (REUTERS) - Major powers are planning to ease sanctions barring trade in gold and other precious metals with Iran in return for steps to shut down its newly expanded Fordow uranium enrichment plant.

Western officials said the offer would be presented to Iran on Feb 26 in Almaty, Kazakhstan, acknowledging that it represented a relatively modest update to proposals that the six major powers put forward last year.

Speaking on condition of anonymity, the officials said their decision not to make a dramatically new offer in part reflected scepticism that Iran was ready to make a deal ahead of its June 14 presidential election.

The group, which includes Britain, China, France, Germany, Russia and the United States - known as the P5+1 - wants Iran to do more to prove that its nuclear programme was for only non-military purposes and to permit wider United Nations inspections.

Iran denies it is seeking nuclear weapons but has refused, in recent years, to halt its uranium enrichment, a process that can produce fuel for nuclear reactors or, ultimately, for bombs.

Israel, which is regarded as the Middle East's only nuclear power and which views a nuclear-armed Iran as an existential threat, has raised the possibility of taking military action to halt the Iranian atomic programme.

While stressing he wants to resolve the dispute with Iran through diplomacy, US President Barack Obama on Tuesday repeated a veiled military threat, saying "we will do what is necessary to prevent them from getting a nuclear weapon".

The core of the new offer revises last year's demand that Iran stop producing higher-grade uranium, ship any stockpiles out of the country and close down its underground enrichment facility at Fordow, near the holy Iranian city of Qom.

"The next proposal is remarkably close to the old one," said one official who spoke on condition of anonymity, describing it as "a way to test whether they are serious or not".

"We don't think the Iranians have given us reason to do much more," he said. "It's basically an update... so it does require a little bit more from Iran in terms of cooperation with the (International Atomic Energy Agency) and at Fordow."

The added inducement for Iran in the new offer is to suspend sanctions on trade in gold and precious metals, something that could be used as part of barter transactions that might allow Iran to circumvent increasingly tight financial sanctions. It goes beyond last year's proposal, in which the powers offered sanctions relief on aviation spare parts, fuel for a medical reactor and other civil nuclear cooperation.

The Western officials described their new proposal as "more for more" - meaning, that they are seeking more steps to curtail Iran's nuclear programme in exchange for greater inducements on their part, but they admitted it is not a dramatic shift.

"It's still more for more, (but) not much more," said the second Western official.