France on Monday said that along with Germany and Britain, it "will not hesitate for a single second to reapply all the sanctions" scrapped a decade ago if European security is threatened by Iran's nuclear activities, as Tehran and Washington are engaged in negotiations for a new agreement.

"Resorting to threats and economic blackmail is entirely unacceptable," Iran's mission to the UN said in a letter carried by the country's ISNA news agency.

France, Germany and Britain, along with China and Russia, are parties to the 2015 nuclear agreement with Iran, which the United States withdrew from three years later during President Donald Trump's first term in office.

Under the agreement, the parties can trigger the "snapback" mechanism, which would automatically reinstate UN sanctions on Iran over its non-compliance, an option that expires in October.

Longtime foes Iran and the United States have been engaged since April 12 in their highest-level talks in years targeting a new deal that would stop Tehran developing nuclear weapons -- an objective it denies pursuing -- in return for relief from sanctions.

Iran's Foreign Minister Abbas Araghchi said last week he was willing to visit Germany, France and Britain for talks.

The Iranian UN mission said in its letter that "genuine diplomacy cannot proceed under threats or pressure".

"If France and its partners are truly interested in a diplomatic resolution, they must abandon coercion."