Talks in Switzerland between international officials and Iran over the Middle Eastern state's nuclear program have been suspended. Both sides are due to draw up a draft deal by March 31.
Six days of discussions in Lausanne were interrupted to allow members of the Iranian delegation to return home to attend the funeral of the president's mother.
While progress has been made between officials from the US, Britain, France, Russia, Germany, China and Iran, relations between nations have been tested by the talks.
Some officials have said France has been pushing for unnecessarily harsh restrictions, while the US has been accused of not pushing hard enough.
The US' Senate Foreign Relations Committee will vote in April on whether President Barack Obama will have to get Congressional approval on any agreement made with Iran.
The re-election of Israeli Prime Minister Benjamin Netanyahu, a vocal critic of any nuclear deal with Iran, has also compounded the pressure on the US delegation.
Western officials have reported the biggest issue in the talks remains Iran's request to have no limits on research and development of advanced centrifuges.
The machines are used to purify uranium for use in nuclear reactors, or even weapons.
The Middle Eastern state is also calling for international sanctions against it to be lifted as soon as a deal is made.
Iran denies allegations from the West that it has the capabilities to produce nuclear weapons.
A 'credible' deal
Posting on social networking site Twitter on Friday, Iranian Foreign Minister Mohammad Javad Zarif accused the Western delegations of asking for too much, saying the choice was between "pressure or agreement."
However US Secretary of State John Kerry was upbeat about the direction the talks were heading, calling them "constructive and productive."
Iran and the six world powers are aiming to have a draft agreement drawn up by the end of the month, followed by a more detailed deal by the end of June.
The talks were also discussed by world leaders at an EU summit being held in Brussels on Friday, with German Chancellor Angela Merkel saying any deal reached must be "credible," and come with assurances Iran won't develop weapons.
Officials say Kerry will meet with his counterparts from Britain, France and Germany in London on Saturday, to discuss a possible deal further.
Talks are due to resume next week.
an/bk (Reuters, AP, AFP, dpa)