WARSAW — Vice President Mike Pence used an American-convened conference on Middle East security to lash out at Washington’s three closest European allies on Thursday, accusing them of trying “to break American sanctions against Iran’s murderous revolutionary regime.”

Mr. Pence delivered his critique of the allies, Britain, France and Germany, in an address at a Warsaw conference organized by the Trump administration. It was a remarkable, open breach with the European nations that, along with the United States, had negotiated the 2015 nuclear accord.

All three nations argued that President Trump made a grave error in abandoning the treaty last year, saying that the Iranians were in compliance even as they continued to conduct missile and space rocket tests that were not covered by the accord.

To the Europeans, it is Mr. Trump — not the Iranians — who was the first to break the agreement’s terms. Since then, they have been trying to persuade the Iranians to continue to comply with the deal, under which Tehran gave up 97 percent of its nuclear material and agreed not to produce any significant quantities of nuclear fuel until 2030. American intelligence agencies told Congress late last month that the Iranians were still in compliance.