Speaking at a conference in Poland attended by Israel and senior Arab leaders, Pence denounced Iran as the "greatest threat to peace and security in the Middle East" and accused the clerical regime of plotting a "new Holocaust" with its regional ambitions.

Pence denounced a new initiative by France, Germany and Britain to let European businesses keep operating in Iran despite renewed US sanctions.

"It's an ill-advised step that will only strengthen Iran, weaken the EU and creates still more distance between Europe and the United States," Pence said.

"The time has come for our European partners to withdraw from the Iran nuclear deal and join with us as we bring the economic and diplomatic pressure necessary to give the Iranian people, the region and the world the peace, security and freedom they deserve," he said.

As Iran's clerical state marks 40 years since the overthrow of the pro-US shah, Pence vowed maximum pressure while not explicitly urging regime change.

"As Iran's economy continues to plummet, as the people of Iran take to the streets, freedom-loving nations must stand together and hold the Iranian regime accountable for the evil and violence it has inflicted on its people, on the region and the wider world," he said.

US sanctions "will get tougher still" unless Iran "changes its dangerous and destabilising behavior," Pence said.

The European Union, including summit host Poland, has shown no sign of rejecting the 2015 accord negotiated under former US president Barack Obama in which Iran constrained its nuclear programme in return for promises of sanctions relief.

EU officials say they acknowledge concerns about Iran but believe the deal, with which Tehran has complied, is working and that the clerical state is not the only problematic actor in the region.