Vice President Mike Pence has accused Washington's European allies of trying to break US sanctions against Tehran and called on them to withdraw from the Iran nuclear deal.
At a Middle East conference organised by the United States in Poland, Pence slammed the European Union for remaining party to the agreement after the Trump administration withdrew from it last year and reimposed tough sanctions on Iran.
"Sadly, some of our leading European partners have not been nearly as cooperative. In fact, they have led the effort to create mechanisms to break up our sanctions," Pence said.
Last month, Germany, France and Britain launched a financial tool to help European firms with legitimate business interests in Iran avoid US sanctions.
Pence said the scheme set up by the EU to facilitate trade with Iran was "an effort to break American sanctions against Iran's murderous revolutionary regime".
"It is an ill-advised step that will only strengthen Iran, weaken the EU and create still more distance between Europe and the United States," he said.
'New era of cooperation'
Earlier, Secretary of State Mike Pompeo called for a "new era of cooperation" in resolving Middle East challenges and said no country could afford to remain on the sidelines.
"The United States seeks a new era of cooperation between all of our countries on how to confront these issues," Pompeo told foreign ministers and other officials from more than 60 countries in Warsaw on Thursday.
"None of the region's challenges will solve themselves. We must work together for security," he said. "No country can afford to remain on the sidelines."
Reporting from Warsaw, Al Jazeera's Hashem Ahelbarra said that while many nations agreed that ending the wars in Syria and Yemen, as well as the Israel-Palestine peace plan, were important, a number of questions remained over the efficacy of the conference, as Russia, China, the Palestinians and Houthis, were not in attendance.
Jared Kushner, US President Donald Trump's son-in-law and adviser, is taking part in the Warsaw conference where he will speak behind closed doors on the contours of a US peace proposal to be presented after Israeli elections in April.
The Palestinian government is not attending and has called the conference an "American conspiracy". It is refusing US mediation after Trump in 2017 recognised bitterly contested Jerusalem as Israel's capital.
On Thursday, a Palestinian official said that the conference lacked credibility as it aimed to "normalise" the Israeli occupation of Palestinian territory.
"By fully siding with the Israeli government, (the Americans) have tried to normalise the Israeli occupation and the systematic denial of the Palestinian right to self-determination," Nabil Shaath, an adviser to Palestinian president Mahmoud Abbas, wrote in a column published by Israeli newspaper Haaretz.
"The Warsaw conference is part of this context," he wrote.
"A peace process cannot be turned into an attempt to obtain amnesty for war crimes or to make one of the parties surrender its basic rights under the UN charter."
The conference is widely viewed as a US-led effort to isolate Tehran, a foreign policy priority of the Trump administration.
Iran was not invited to the meeting, despite being a "key player" in the region, Ahelbarra explained.
"We're back to square one where it is all about Iran," he added.
The European allies of Washington have voiced concern that the conference would turn into an Iran-bashing session and increase tensions with Tehran. The European Union's top diplomats stayed away from it.
Last year, the US unilaterally withdrew from the Iran nuclear deal and reimposed sanctions on Tehran, but European countries have continued to support the multilateral agreement as the best way to deal with Iran's nuclear programme.
Earlier on Thursday, in a meeting with Israeli Prime Minister Benjamin Netanyahu, Pompeo said peace and stability in the Middle East could not be achieved without confronting Iran.
Iran says its ballistic missile programme is defensive and a deterrent only, and that it has deployed forces to Syria and Iraq only after the invitation of governments of those countries.
Iran's Foreign Minister Mohammad Javad Zarif said at a news briefing on Wednesday in Tehran that the two-day Warsaw meeting was "dead on arrival".
Zarif described the meeting as the "Warsaw Circus", and said it was "no coincidence that Iran is hit by terror on the very day" the talks began in the Polish capital.
The Warsaw meeting brought together Israel and some Gulf Arab countries, including Saudi Arabia.
Israel's prime minister, who met the Omani foreign minister on the sidelines on Wednesday, called the conference a "historical turning point" in tackling the threat from Iran.
"I think this marks a change, an important understanding of what threatens our future, what we have to do to secure it and the possibilities of cooperation that extend beyond security to every realm of life for the peoples of the Middle East," said Netanyahu, who faces an election on April 9.