"We're moving forward," Iranian Foreign Minister Mohammad Javad Zarif told reporters after meeting separately with his German and French counterparts, who he said were both "serious" about reaching an agreement.
"I think we can in fact make the necessary progress to be able to resolve all the issues and start writing them down in a text that will become the final agreement once it's done," Zarif said as the clock ticked down to Tuesday's deadline for the broad outlines of a deal.
France was "insisting" any deal included mechanisms to ensure that the Islamic republic, which denies wanting nuclear weapons, complies with its commitments, he said.
Asked later after meeting Zarif and separately with top US diplomat John Kerry if progress was being made, Fabius said: "We're working, we're working. We're trying to make progress."
German Foreign Minister Frank-Walter Steinmeier also arrived in Lausanne saying the talks had entered their "endgame" after 12 years, but warning also this would also be the hardest stage.
The powers want Iran to shrink its nuclear programme in order to make it easy to detect any dash to make a bomb under the guise of its civilian atomic programme.
In return, Iran wants an easing of international sanctions that have excluded the Islamic republic from lucrative oil markets and crippled its economy.
Asked at the start of their talks Saturday whether they were expecting a good day, Kerry replied wryly that "we're expecting an evening today," while Zarif joked "evening, night, midnight, morning."
