"We are certainly concerned about these provocative announcements," President Joe Biden's press secretary, Jen Psaki, told reporters.

"We believe that the diplomatic path is the only path forward here and that having a discussion, even indirect, is the best way to come to a resolution."

Iran announced it was accelerating its production of enriched uranium after damage was inflicted on the Natanz nuclear plant in a mysterious incident that Tehran blames on Israeli sabotage.

The incident has cast a shadow over attempts by Washington to negotiate a renewal of the so-called JCPOA deal, which brought Iran's nuclear power industry under international scrutiny but which Donald Trump abandoned in 2018.

Indirect talks between Washington and Tehran are underway in Vienna, and Psaki said she expects them to continue.

The six countries that negotiated the JCPOA with Iran should be "unified in rejecting" Iran's threat to accelerate uranium enrichment, Psaki said.

However, it "underscores the imperative of returning to mutual compliance with the JCPOA," she said, calling talks last week "constructive."

"While (talks) were difficult and while we expect this to be long, we expect and we have not been alerted of any change in planned attendance in meetings that will resume later this week."