The United States believes Iran's breakout time to producing enough highly enriched uranium for one nuclear weapon is now "really short" and alarming.

A senior Biden administration official told reporters on Friday, speaking on condition of anonymity, did not have a precise time for the breakout, which has been estimated to be a matter of months.

"But it's really short. It is unacceptably short," the official said, calling it "alarming".

US National Security Advisor Jake Sullivan said later that the nuclear talks are "not going well" and US has communicated to Iran through Europeans its "alarm" over Iran's expanding nuclear program.

The official said Andrea Gacki, the Treasury Department's director of foreign asset control, was in the United Arab Emirates earlier this week urging private companies not to evade sanctions against Iran.

"If you are evading sanctions, the U.S. will have its eye very much on you. There will be consequences," the official said.

Iran last month said it has stockpiled well over 200 kilograms of 20-percent and 60-percent enriched uranium, and continues purification of the fissile material. Ninety-percent enrichment is needed to use the material for producing a bomb, but the step from 60-percent enrichment to usable grade is relatively short.

With reporting by Reuters