Marathon talks towards an Iran nuclear deal are gathering pace with US President Barack Obama appealing to Tehran to seize an "historic" opportunity.
In Switzerland, US Secretary of State John Kerry and his Iranian counterpart began a fifth day of discussions on Friday, while in Brussels the leaders of Germany, France and Britain also met.
In a Persian New Year video address, Obama said a "reasonable nuclear deal ... can help open the door to a brighter future for you the Iranian people."
"I believe our nations have an historic opportunity to resolve this issue peacefully - an opportunity we should not miss."
In an apparent response, Iranian Foreign Minister Mohammad Javad Zarif said it was the other side that had to make a decision.
"Iranians have already made their choice: engage with dignity.
"It's high time for the US and its allies to chose: pressure or agreement," Zarif wrote in a message posted on his official Twitter account.
Iran and six world powers - the US, China, Russia, Britain, France and Germany - are seeking to agree the outlines of a deal by March 31 after more than a year of negotiations.
The mooted agreement, due by July, is aimed at convincing the world after a stand-off now in its 13th year that Iran won't build nuclear weapons under the guise of its civilian program.
The complex deal would likely involve Iran reducing nuclear activities, allowing tight inspections, exporting nuclear material and limiting development of nuclear machinery.
In exchange Iran, which denies wanting nuclear weapons, would be granted staggered relief from painful sanctions that have strangled its oil exports and hammered its economy.
Obama's Republican opponents and Israel's freshly re-elected Prime Minister Benjamin Netanyahu, whose country is assumed to have nukes itself, fear the deal will not do enough to stop Iran getting the bomb.
Negotiators missed two deadlines last July and November for a deal but the pressures in Washington - where Republicans are teeing up new sanctions legislation - all but rule out a new extension, experts say.
Both Kerry and Zarif on Thursday spoke of "progress" in the talks but both sides have said there remain considerable gaps still to bridge.
"I think we are pretty far from a deal," a European negotiator involved in the talks in Swiss city of Lausanne said on Thursday.
"The Iranians go back, go forward, it changes every day," the diplomat said in Lausanne.
As a result it was unclear whether the current round of talks would wrap up as tentatively scheduled on Friday and resume next week, or continue indefinitely.
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