US officials said that a nuclear deal with Iran was in sight by a March 31 deadline, as top US diplomat John Kerry arrived in Switzerland for down-to-the-wire talks.
The landmark agreement -- which would cap more than a decade of painstaking negotiations -- is designed to stop Iran from developing a nuclear bomb in return for the lifting or easing of crippling economic sanctions.
"We very much believe that we can get this done by the 31st. We see a path to do that," a senior State Department official told reporters, cautioning that it was still possible the political framework to rein in Iran's nuclear program could elude them at the last.
The official added: "We can see a path forward here to get to an agreement, we can see what that path might look like... that doesn't mean we'll get there."
Kerry is set to meet in the Swiss city of Lausanne on Thursday with Iranian Foreign Minister Mohammad Javad Zarif, for what he hopes will be the successful culmination of months of closed-door negotiations.
Beyond Thursday there was "a blank slate" with all days reserved for further talks and an "incredibly fluid" schedule, the US official said.
"We made more progress in the last round (last week) than we have made the previous rounds, which often happens once you're getting closer to a deadline."
It was not immediately clear when other ministers from the P5+1 group of world powers were to arrive in Switzerland but, as the nuclear deal inches closer they were expected to arrive to give a green light, if the talks get that far.
The US official admitted that US officials in Washington and Lausanne were prepared for next steps if the March 31 deadline is missed.