Kerry late last year said the focus for March was agreement on "the major elements" of a comprehensive deal that would set a "clear path" for a June deal. If that failed to materialize, "we can revisit how we then want to choose to proceed," he added.

Obstacles remain on several main issues - uranium enrichment, where stockpiles of enriched uranium should be stored, limits on Iran's nuclear research and development and the timing and scope of sanctions, among other issues, according to negotiators.

In Israel, Prime Minister Benjamin Netanyahu renewed his severe criticism of the unfolding deal, saying it would leave intact much of Iran's nuclear infrastructure, including underground research facilities, a plutonium reactor and advanced centrifuges capable of enriching uranium.

The US says any final deal will accomplish a goal of stretching the time Iran would need to make a nuclear weapon from several months to a year. But Netanyahu said Washington initially promised "years" to a breakout time.

"In our estimate, it will be reduced to perhaps a year, most likely much less than that," he said.

Kerry and others, including Mohammad Javad Zarif, the Iranian foreign minister, have said the sides have made some progress. Other officials have said Iran is considering demands for further cuts to its uranium enrichment program but pushing back on how long it must limit technology it could use to make atomic arms.

Tehran says it wants to enrich only for energy, science, industry and medicine. But many countries fear Iran could use the technology to make weapons-grade uranium.