US sets 'bottom lines'

The United States on Friday set out what it called its "bottom lines" to reach a deal with Iran to rein in its nuclear program, ahead of new talks next week.

Washington had stuck to its guns that it wanted a "good deal" and had agreed to several extensions of the negotiations "because we have held firm to certain bottom lines," a senior US administration official said.

US Secretary of State John Kerry will leave at the weekend for Switzerland, where he will meet once again with his Iranian counterpart Mohammad Javad Zarif.

The pace and intensity of the talks have sped up as global powers gathered under the P5+1 group have been seeking a deal with Iran to stop it acquiring a nuclear weapon -- something Tehran denies trying to do.

A March 31 deadline for a political framework for the deal is looming with negotiators saying they will aim to pin down the final technical details by June 30.

While US officials cautioned that there were no guarantees a deal would be reached, they said "the negotiations have advanced substantially, gaps have narrowed."

The officials were talking only days ahead of a controversial visit to Washington by Israeli Prime Minister Benjamin Netanyahu, who has vehemently opposed a nuclear deal with Iran.

Netanyahu is due to address the US Congress on Tuesday on why he believes the Iran deal is a bad move, despite opposition from the White House which sees it as interference ahead of Israel's elections later in March.