Senior European officials on Monday castigated U.S. President Donald Trump's renewed sanctions on Tehran as “illegal” and in violation of a U.N. Security Council resolution, and they vowed to intensify efforts to thwart the U.S. measures and preserve the Iran nuclear accord.

The effort to preserve the nuclear deal, led by the EU and by the three European architects of the accord — France, Germany and the U.K. — puts the United States in direct conflict with its largest and most powerful NATO allies. It represents the sharpest break between Washington and its European partners on foreign policy since Trump took office and began calling into question decades of diplomatic norms.

Trump unilaterally withdrew from the nuclear accord, formally known as the Joint Comprehensive Plan of Action (JCPOA), in May. Other parties to the accord — the Europeans, as well as Russia and China — have repeatedly reaffirmed their commitment to it, citing multiple inspections by the International Atomic Energy Agency showing Iran to be in compliance.

A first battery of U.S. sanctions against Iran was set to snap into effect at midnight Monday, with other sanctions, notably against Iran's oil industry — the lifeblood of its economy — and its energy sector in general to be reimposed in November.

In a statement, Trump promised tough enforcement and warned violators of "severe consequences."

In Brussels, senior diplomats issued a stern statement expressing "regret" at the U.S. action.

"The United States is fully committed to enforcing all of our sanctions, and we will work closely with nations conducting business with Iran to ensure complete compliance," Trump said. "Individuals or entities that fail to wind down activities with Iran risk severe consequences."

In two briefing calls for reporters, senior Trump administration officials insisted that the nuclear deal, negotiated by Barack Obama, has not gone far enough to curtail Iran's nuclear weapons program or to stop Tehran from other aggressive actions in the Middle East and beyond, including sponsorship of terrorism.

"This administration is fully committed to rigorously enforcing our sanctions and ensuring the Iran has no path to a nuclear weapon," one senior administration official said. Another official added: "The purpose of a robust sanctions enforcement is to deny the regime the revenues it uses to finance terrorism and other malign programs around the world."

In Brussels, senior diplomats issued a stern statement expressing "regret" at the U.S. action and vowing to uphold the JCPOA. The EU announced that a "blocking statute" intended to shield European companies that continue doing business with Iran from so-called secondary sanctions would take effect at precisely the same time — 6 a.m. Tuesday in Europe.

'Free enterprise'

At a news conference, senior European Commission officials bluntly accused the Trump administration of violating international law, and said their blocking statute would allow EU companies that continue doing business in Iran to recover any damages, including legal costs, from entities — including U.S. banks and businesses — acting to enforce the U.S. sanctions.

"The EU is a market economy, we protect freedom of enterprise," a senior Commission official said. "The purpose of the blocking statute is not to oblige any company to invest but actually it is to make sure their business decisions remain free and not imposed upon by legislation, which we consider unlawful."

Another senior official added, "It's very important to recall that the JCPOA is part of the U.N. Security Council Resolution 2231, it is fully part of it, so it is a U.N. Security Council resolution that has been adopted and has the strength that we know in international law."

Political leaders were also forceful in criticizing Washington.

“We deeply regret the re-imposition of sanctions by the U.S., due to the latter’s withdrawal from the Joint Comprehensive Plan of Action,” the EU’s high representative for foreign affairs, Federica Mogherini, said in a statement also signed by the foreign ministers Jean-Yves Le Drian of France, Heiko Maas of Germany, and Jeremy Hunt of the United Kingdom.

“The JCPOA is working and delivering on its goal, namely to ensure that the Iranian program remains exclusively peaceful, as confirmed by the International Atomic Energy Agency,” they said.

The most direct conflicts may not come until November, when U.S. sanctions intended to cripple Iran's oil industry come back into effect. European officials have already said that they are working to find ways to keep Iran's oil business alive, including new financing mechanisms perhaps through European central banks. China has already said it will continue buying oil from Iran.

But Washington is adamant it will stop the oil revenues that are financing the Iranian regime, including its military activities in Syria, Lebanon, Yemen and elsewhere, in addition to its support for groups like Hezbollah. "It is our policy to get as many other countries to zero as possible," a senior Trump administration official said of Iranian oil imports.

Risk to civilians

In their briefings to reporters, U.S. officials also reiterated Trump's willingness to meet Iranian President Hassan Rouhani — without any preconditions.

"The president has been very clear: None of this needs to happen," an official said. "He will meet with the Iranian leadership any time.” But the Americans also lambasted the Iranian regime for victimizing its own people, violating human rights and sponsoring terrorism. They also pushed back hard on questions about sanctions related to Iran's civilian aviation sector — and the potential risks to ordinary citizens flying on unsafe aircraft — by saying that Iran's government frequently uses its commercial aviation sector for military purposes.

European officials, under pressure for details about how their blocking statute would work in practice, acknowledged that their legislation would not fully blunt the impact of U.S. sanctions. But they said they want to provide as much reassurance as possible to European businesses. They also said there is continuing strong solidarity with Russia and China to preserve the economic benefits in the JCPOA that convinced Tehran to curtail its nuclear weapons program.

European officials noted that the U.S. has rarely tried active enforcement of secondary sanctions.

"We're doing our best," a third senior EU official said. "The U.S. has sent a very strong signal to the markets, and now we are trying to reverse that signal."

Some experts say the EU’s moves are unlikely to have the desired effect, arguing that the blocking statute would create legal burdens for Europe-based companies without preventing the U.S. from targeting their American branches and assets. For many companies, the risk of being cut off from business in the U.S. — a far bigger market than Iran — is enough to make them want to comply with Washington’s demands.

However, European officials noted that the U.S. has rarely tried active enforcement of secondary sanctions and had met with little success in the few attempts in the past. The Europeans said they are hoping for a political deal as occurred in the 1990s during a similar dispute over unilateral U.S. sanctions against Cuba and Iran.

"It's really uncharted waters," the third senior official said. "Last time we had a political settlement. Let's see if we'll have a political settlement."

Nahal Toosi contributed reporting.