LAUSANNE, Switzerland�French Foreign Minister�Laurent Fabius�on Saturday said he hopes a �robust� nuclear agreement can be sealed with Iran but that any deal must include real transparency on Iran�s future nuclear activities and a control mechanism to ensure Tehran sticks by its promises.

Mr. Fabius and German Foreign Minister�Frank Walter Steinmeier�joined the nuclear talks in Lausanne, Switzerland on Saturday morning ahead of�a March 31 deadline to seal a political understanding for a nuclear deal. Iran and the group of six global powers it negotiates with have set a June 30 deadline for a final, detailed accord.

U.S. Secretary of State�John Kerry�has�already held two days of talks�here with Iranian Foreign Minister�Javad Zarif�and other top officials. A senior U.S. official described those talks on Friday as tough and very serious.

On Friday, British Foreign Secretary�Philip Hammond�suggested�meeting the March 31 deadline could be challenging�and that talks on a political agreement may drift into the first half of April.

Mr. Fabius has�adopted a strong line in the Iran talks, with France appearing at odds with the U.S. at times on what a strong agreement must contain.

Speaking to reporters outside the hotel where the talks are taking place, Mr. Fabius said �I come here with the wish to advance towards a robust accord.�

�The discussions have been long, difficult. We advance on some points and on other points not enough,� he added.

He said what is very important is the �transparency� Iran agrees to for overseeing its nuclear activities and the �controls, to be sure that the commitments made are respected.�

Mr. Steinmeier said that after 12 years of nuclear talks with Iran, negotiations have entered the �endgame.� However, he said the final steps to be taken �are the most difficult but also the decisive ones.�

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

�I can only hope that given what we have achieved in the last 12 months that we don�t cease to try and reach a final agreement. The last 12 months have shown that there is serious willingness on all sides to negotiate,� he said.

French officials have stressed several specific points in recent weeks. They want to be sure there are real constraints on Iran�s nuclear research work to ensure Tehran cannot advance toward more sophisticated technology, which would allow them to produce nuclear fuel much more quickly.

They have stressed the importance of full access to sites where Iran could hide nuclear-related work and they want to be sure Tehran addresses long-standing questions about its past nuclear work that western officials believe was aimed at gaining nuclear weapon know-how.

Iran says its nuclear program has always been for civilian purposes.

U.K. Foreign Secretary Philip Hammond is expected to arrive in Lausanne on Sunday as is Federica Mogherini,�the European Union foreign policy chief who formally chairs the six power group.

Iran negotiates with the U.S., the U.K., France, Russia, China and Germany on its nuclear program.

In return for steps to block its path to a nuclear weapon, tight international sanctions on Iran will be eased.

Among the other issues still to be resolved are the timing of sanctions relief for Iran and specific limits on how much enrichment Iran can carry out under a nuclear agreement.

It remains unclear whether Iran and the six powers will seek to hammer out a written deal by March 31 setting out the political contours of a deal.

Mr. Hammond suggested on Friday that may not happen although U.S. lawmakers are pressing for clear indications that Iran has agreed to real constraints on its nuclear activities by March 31 to hold off fresh sanctions bills which could scuttle the diplomacy.

This article was written by�Laurence Norman for The Wall Street Journal on Mar. 28, 2015.