"We reject any ultimatums and we will assess Iran's compliance on the basis of Iran's performance regarding its nuclear-related commitments" under a 2015 deal, the EU's foreign policy chief Federica Mogherini and the foreign ministers of Britain, France and Germany said in a statement.

Iran said Wednesday it had stopped respecting limits on its nuclear activities agreed under the deal with major powers until they find a way to bypass renewed US sanctions, as Washington accused Tehran of resorting to "blackmail".

Tehran said it was responding to the sweeping unilateral sanctions that Washington has reimposed since it quit the agreement one year ago, which have dealt a severe blow to the Iranian economy.

In their statement, the Europeans underlined "our own firm commitments under the agreement including as regards sanctions-lifting for the benefit of the Iranian people" but criticised the US sanctions.

"In this regard, we regret the re-imposition of sanctions by the United States following their withdrawal from the JCPoA," -- Joint Comprehensive Plan of Action -- as the nuclear deal is known.

"We call on countries not party to the JCPoA to refrain from taking any actions that impede the remaining parties' ability to fully perform their commitments."

The European powers added that they were "determined to continue pursuing efforts to enable the continuation of legitimate trade with Iran" in an effort to keep the imperilled pact afloat.

But it said that Iran must at the same time "implement its commitments under the JCPoA in full as it has done until now and to refrain from any escalatory steps".