The foreign ministers of France and Germany joined the international talks on Iran’s nuclear programme on 28 March as the negotiations in Lausanne, Switzerland, move into the final stage.

The new arrivals joined Iranian Foreign Minister Mohammad Javad Zarif and US Secretary of State John Kerry, who resumed bilateral discussions on 26 March.

Iran and the P5+1 world powers are attempting to reach an agreement by a 31 March deadline that would put limits on Tehran’s nuclear development programme in exchange for relief from international economic sanctions.

“In negotiations, both sides must show flexibility,” Zarif said on his official Twitter account. “We have, and are ready to make a good deal for all. We await our counterparts’ readiness.”

US officials have said little about the progress of the discussions since Kerry’s arrival in Switzerland. 

“The serious but difficult work continues,” a senior US State Department official was quoted as saying by Reuters. “We expect the pace to intensify as we assess if an understanding is possible.”

The British, Chinese and Russian foreign ministers are due to arrive in Lausanne on 29 March, along with Federica Mogherini, the leader of foreign policy for the EU.

The meeting takes place 18 months after the breakthrough interim agreement was hammered out in Geneva in November 2013.

The most recent multilateral talks ended in November 2014 with several differences remaining between the two sides’ positions, including: the number of centrifuges Tehran will be allowed to keep in its uranium enrichment programme; the level of relief from international sanctions it will receive after the initial agreement; and the duration of the deal.

If a political agreement can be reached by 31 March, the negotiators will then work to reach a fully detailed deal by July.

Iranian President Hassan Rouhani has reportedly sent a letter to the heads of state of all six negotiating powers expressing the need for a resolution to the nuclear issue. He also spoke to five of the six leaders, not including US President Barack Obama.

In recent weeks, France has downplayed the need to rush talks to meet the end-of-March deadline. French diplomats have reportedly insisted that sanctions could only be lifted if Iran explains evidence of historical nuclear warhead design.

Tehran hopes that a long-term agreement on its nuclear programme will include the lifting of all economic sanctions imposed by the US, EU and UN, which have crippled Iran’s economy in recent years.

The World Bank estimates that Iran’s real GDP contracted by 5.8 per cent in 2012/13 and by a further 1.7 per cent in 2013/14, based on the Iranian calendar starting and ending in March.

The end of sanctions could drive significant overseas investment in the country’s ailing oil and gas industry. Iran has announced plans to tender 17 oil and gas blocks to international oil companies (IOCs), but there is little interest while sanctions remain in place.