Nuclear negotiations between the United States and Iran are entering a critical phase with U.S. Secretary of State John Kerry meeting his Iranian counterpart just days away from an end-of-month deadline for the outline of a deal.

Less than a week before that target, Kerry and Iranian Foreign Minister Mohammad Javad Zarif and their teams huddled Thursday in the Swiss resort town of Lausanne.

U.S. officials said the March 31 deadline is achievable but remains uncertain amid significant gaps.

One official said the American side "can see a path forward to get to agreement" by the end of March as the last round of talks produced more progress than many previous rounds. The official was not authorized to discuss the talks by name and spoke on condition of anonymity.