UN nuclear inspectors have arrived in Iran for more talks with officials about the country's controversial nuclear programme.
It is the second visit in a month by the International Atomic Energy Agency (IAEA), reflecting growing international concerns about alleged weapons experiments.
Chief inspector Herman Nackaerts said his "highest priority" was to clarify the "possible military dimensions" of the nuclear programme, although he warned it "may take a while".
Iran insists its uranium enrichment work is peaceful and will be used primarily to generate electricity.
However, Western nations believe the programme is geared towards making weapons and is a major threat to the stability of the region.
Tensions have risen over speculation that Israel may carry out a military strike on Iran's nuclear facilities.
US National Security Adviser Tom Donilon has flown to Israel for discussions with Prime Minister Benjamin Netanyahu and other senior officials.
But the head of the US Joint Chiefs of Staff, General Martin Dempsey, said on Sunday it was still unclear whether Iran had the ability to make a nuclear bomb.
"On that basis, I think it would be premature to exclusively decide that the time for a military option was upon us," he said.
Foreign Secretary William Hague has warned Israel that military action against Iran would not be "a wise thing".
He said economic sanctions and negotiations had to be given "a real chance" to convince Tehran to abandon its nuclear ambitions.
In a television interview Mr Hague also repeated his warning that if Iran built nuclear weapons it could result in another Cold War - this time in the Middle East.
"They would either be attacked and there would be a war, or there would be a cold war in which Iran would be subject to these very intense economic sanctions and they would find that other nations in their region developed nuclear weapons," he said.
Last week Iranian President Mahmoud Ahmadinejad took part in an elaborate ceremony to show how much progress had been made with the country's nuclear programme.
Tehran said it had used domestically-made nuclear fuel in a reactor for the first time, as well as developing faster, more efficient uranium enrichment centrifuges.
State TV showed Mr Ahmadinejad inspecting the fuel rods as they were loaded into a reactor.
The US and the EU have tightened sanctions against Iran after the IAEA said last November it may have carried out tests "relevant to the development of a nuclear explosive device".
Iran said on Sunday it had halted oil sales to British and French companies, ahead of a European Union oil embargo set to begin on July 1.
The price of crude rose to an eight-month high following the announcement.
Iran has since threatened to extend the oil ban to other EU countries.
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