Iran negotiations in 'endgame': German Foreign Minister

German Foreign Minister Frank-Walter Steinmeier on Saturday joined nuclear talks with Iran in Switzerland, saying the negotiations were in the "endgame" as the negotiations near a critical 1 April deadline.

"After 10 or almost 12 years of talks with Iran, the endgame of the lengthy talks, so to speak, is beginning here," Steinmeier told reporters before joining his US, French and Iranian counterparts in Lausanne ahead of a Tuesday deadline to agree the framework of a deal.

"And here, with a view of the Swiss mountains, I'm reminded that as one sees the cross on the summit, the final metres are the most difficult but also the decisive ones," he said.

"That's what has to be done here in the coming hours and days. I can only hope that in view of what has been achieved over the last 12 months that the attempt for a final agreement here will not be abandoned."

He added that a successful conclusion of the nuclear talks with Iran "could perhaps bring a bit more calm" to the Middle East.

Steinmeier joined US Secretary of State John Kerry, Iranian Foreign Minister Mohammed Javad Zarif and negotiators from the six powers, chasing an agreement on the broad outlines of what they hope will be a historic deal by Tuesday.

"We're at that point in the negotiations where we really need to see decisions being made," a senior US State Department official said late Friday at the talks in Switzerland.

"The work is very complicated and difficult. The other side needs to choose between pressure and a political accord," countered Zarif.

Iranian President Hassan Rouhani on Thursday reportedly sent a letter to the leaders of the P5+1 group - the US, the UK, France, China and Russia, plus Germany- about the nuclear talks.

"We have confirmed that a letter was passed to the US negotiating team that was intended for President Obama and was sent by President Rouhani," White House spokesman Josh Earnest said Friday, declining to give any details about content.

He noted that the White House team and the president himself are aware of the content of the letter.

The Iranian leader also made a series of phone conversations with Russian, Chinese, British and French leaders, to convey Tehran's stance. 

Sharing his comments on his Twitter account, the Iranian president termed the talks as "a golden opportunity" to find a permanent solution and noted that he reiterated "the necessity to move forward based on mutual respect and equal footing."

Kerry is under pressure to return from Lausanne with something concrete to head off a push by Republican lawmakers to introduce yet more sanctions, potentially torpedoing the whole negotiating process.

The Republicans are concerned that by leaving some of Iran's nuclear infrastructure intact, as seems likely, the mooted deal will not do enough to prevent Iran getting the bomb. Iran has always denied wanting to build a nuclear bomb.  

This concern is shared by Israel, widely assumed to have nuclear weapons itself, as well as Saudi Arabia, worried by the prospect of any US-Iranian rapprochement with the West after 35 years of acrimony.

"A deal is possible, but Iran will have to make painful choices," a Western diplomat said, adding however that "the Iranians like to negotiate on the edge of a precipice. They're very good at it."