London, UK: Iran's top leader voiced mistrust on Saturday of US efforts to reach a nuclear deal, even as Washington and its allies spoke of real progress and urged Tehran to take ‘difficult decisions’.

With just 10 days remaining until an end-of-March deadline for a framework agreement, Iran's Supreme Leader Ayatollah Ali Khamenei denounced US ‘bullying’ in the negotiations and repeated Tehran's denial that it was seeking to develop a nuclear weapon.

After week-long talks with Iran in Switzerland, US Secretary of State John Kerry flew to London to confer with his counterparts from Britain, France and Germany on prospects for resolving the dispute, which goes back more than a decade and has threatened at times to unleash a new war in the Middle East.

In a joint statement, the foreign ministers said they agreed that ‘substantial progress’ had been made with Iran in key areas, but some important issues remained outstanding.

‘Now is the time for Iran, in particular, to take difficult decisions,’ they said.

British Foreign Secretary Philip Hammond told reporters the Western ministers were all in agreement that ‘we will not do a bad deal that does not meet our red lines’.

Together with China and Russia, the four countries are trying to reach a deal with Tehran that would restrict the most sensitive aspects of Iran's atomic program in return for an easing of international sanctions.

US ally Israel views Iran's nuclear activities as an existential threat, despite Tehran's strenuous denials that it wants to acquire nuclear weapons.

US President Barack Obama said in an interview with the Huffington Post that Iran had not yet made the necessary concessions for a deal to get completed.

‘What is going to have an effect on whether we get a deal done is, number one: is Iran prepared to show, to prove to the world, that it is not developing a nuclear weapon, and can we verify that in an intrusive, consistent way?’ Obama said.

‘And frankly, they have not yet made the kind of concessions that are I think going to be needed for a final deal to get done. But they have moved, and so there's the possibility,’ Obama added in the interview conducted on Friday and published on Saturday.

Durable solution

After 2-1/2 hours of talks in London, the Western ministers said in their joint statement: ‘Any solution must be comprehensive, durable and verifiable. None of our countries can subscribe to a deal that does not meet these terms.’

The stress on unity seemed designed to counter the impression of a split between Washington and Paris.

US officials have privately bristled at France’s outspoken criticism of the negotiating process and its demands for more stringent restrictions on the Iranians. Officials have expressed concerns that the French might block a deal at the United Nations.

France's envoy to Washington, Gerard Araud, has been especially vocal, tweeting that setting a March 31 deadline for a framework deal was ‘a bad tactic’ and ‘counterproductive’. The target date for a full agreement is 30 June.

The full six-power group is due to resume negotiations with Iran next week in Lausanne, Switzerland.

The West suspects Iran of seeking the ability to produce atomic weapons and the United Nations has imposed stringent economic sanctions on Tehran. Iran says its program is intended only for peaceful purposes, such as medical technology and nuclear energy, and wants the swift lifting of sanctions.

Iran's Khamenei, who has the last word on all matters of state, reiterated in a speech that Tehran would not be pressured into giving in to Western demands.

‘What the Iranian people don't want is imposition and bullying from America,’ he said, at one point repeating the phrase ‘Death to America’ after it was shouted from the crowd.

He accused Washington of seeking to foment instability in the Middle East.

‘They raise the issue of an atomic bomb. They know themselves that we are not pursuing nuclear weapons. But they just use that as an excuse to pressure the Iranian people.’

He criticised ‘arrogant’ Western countries for what he said was their role in bringing about a halving of world oil prices that has squeezed Iran's economy.

‘They insist on putting pressure on our dear people’s economy. What’s their goal? Their goal is to put the people against the system,’ he said.

Iranian President Hassan Rouhani had struck a more positive note earlier, saying there was ‘nothing that cannot be resolved’.