A senior US administration official has said it would be "very hard", though still possible, for six world powers to reach an initial nuclear deal with Iran in talks in Geneva.
"I think we can (get a deal), whether we will, we will have to see because it is hard. It is very hard," the official told reporters after talks between the six and Iran resumed in the Swiss city.
The official also said that the vast majority of sanctions on Iran would remain in place after any preliminary accord on limiting its disputed nuclear programme, and that Washington would "vigorously" implement them.
The five permanent UN Security Council members plus Germany are meeting with Iran to negotiate a deal that would lead to the freezing of some sensitive parts of Tehran's nuclear program.
That would be in exchange for temporary relief from international sanctions that have crippled the Iranian economy.
It is the third round of Iran negotiations in Geneva since October.
"These negotiations are difficult," the official said. "They are tough. There are moments of tension. There are moments of even humour, occasionally."
Responding to US and Israeli critics of the proposed sanctions relief for the Islamic Republic, the official said the proposed relief on offer to Tehran is "quite small."
The proposed measures would include allowing Iran access to some frozen funds in overseas accounts in instalments, the ability to trade in some precious metals and a possible temporary lifting of pressure on countries not to import Iranian oil.
The official said that it would be best for public figures in Iran and the United States to refrain from rhetoric that deepens the mistrust between the two estranged nations, which have not had diplomatic relations since 1980.
The US, Russia, China, France, Britain and Germany came close to winning concessions from Iran on the scope of its nuclear work in return for some sanctions relief at negotiations in Geneva earlier this month.
But diplomats caution that differences remain and could still prevent an agreement during the talks over the next three days.
The last meeting, which ended on 9 November, stumbled over Iran's insistence that its "right" to enrich uranium be recognised.
There was also disagreement over its work on a heavy-water reactor near Arak, which could yield plutonium for atomic bombs once it becomes operational.
Iranian Foreign Minister Mohammad Javad Zarif has since indicated a way around the first sticking point, saying Tehran has the right to refine uranium but is not insisting others recognise that right.
A UN nuclear watchdog report last week showed Iran had stopped expanding its enrichment of uranium and had not added major new components at Arak since August, when moderate Hassan Rouhani replaced hardliner Mahmoud Ahmadinejad as president.
Mr Zarif said on the eve of the meeting there was "every possibility" of a successful conclusion provided there was good faith and the political will among all involved to resolve problems.
Mr Obama sounded a more cautious note, however, saying it was unclear whether the world powers and Iran will be able to reach an agreement soon.
"We don't know if we'll be able to close a deal with Iran this week or next week," he told a Wall Street Journal forum.
The talks are expected to resume with a meeting between Mr Zarif and European Union foreign policy chief Catherine Ashton.
Western governments suspect Iran has enriched uranium with the covert aim of developing the means to fuel nuclear weapons, a charge Tehran denies.
Refined uranium can fuel nuclear power plants - Iran's stated goal - but also provide the core of a nuclear bomb, if enriched further.