UN nuclear inspectors have arrived in Tehran for the second time in a month to discuss Iran's nuclear programme.
Chief inspector Herman Nackaerts said his team's "highest priority" was to clarify the "possible military dimensions" of the nuclear programme.
But he cautioned that progress "may take a while".
Iran insists its uranium enrichment work is peaceful in purpose, but Western nations believe the programme is geared towards making weapons.
Tensions have risen over speculation that Israel may carry out a military strike on Iran's nuclear facilities.
US National Security Adviser Tom Donilon arrived in Israel at the weekend for talks with Prime Minister Benjamin Netanyahu and other senior officials.
But the head of the US Joint Chiefs of Staff, General Martin Dempsey, warned on Sunday that it was still unclear whether Iran was at a stage to assemble 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," Gen Dempsey said.
Last week Iranian President Mahmoud Ahmadinejad took part in an elaborate ceremony to unveil new developments in his 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 the president inspecting the fuel rods as they were loaded into a reactor.
The IAEA inspectors described their last visit, in January, as positive, and said Iran was "committed" to "resolving all outstanding issues".
Mr Nackaerts said on Sunday that he hoped to have a "couple of good and constructive days in Tehran".
"Importantly we hope for some concrete results from the trip. The highest priority remains of course the possible military dimensions of Iran's nuclear programme, but we want to tackle all outstanding issues," he said.
"This is of course a very complex issue that may take a while. But we hope it can be constructive".
The inspectors' evaluation of their visits may form part of the next written report on Iran's nuclear programme, expected later in February.
Tehran says its nuclear activities are simply for electricity generation.
But last November, the IAEA said it had information suggesting Iran had carried out tests "relevant to the development of a nuclear explosive device".
That information led to a decision by the US and the European Union to tighten sanctions against Iran, including measures targeting the country's lucrative oil industry.
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 1 July. Analysts say this gesture of retaliation is largely symbolic, as neither the UK nor France import a large proportion of their oil from Iran.