Congress and the Trump administration are issuing a stern warning to European partners: End all business ties with the Iranian regime or face harsh new sanctions in the coming months, a move that could impact international financial markets and U.S. banks tied to foreign monetary institutions, according to multiple senior U.S. officials who spoke to the Washington Free Beacon about diplomatic efforts to pressure Europe on Iran.

The Trump administration and many in Congress have been working in tandem to pressure European partners over their ongoing financial ties to Iran ahead of efforts by the United States to fully reinstate harsh sanctions on Iran, including on its lucrative oil sector and banking systems, U.S. officials said.

Key European partners have expressed opposition to the new U.S. sanctions in recent months and have held a series of meetings with senior Iranian officials to discuss tactics for evading the new American sanctions, which follow on the heels on President Donald Trump's decision to abandon the landmark nuclear deal.

The increased financial pressure if part of a larger effort by the Trump administration and its allies in Congress to further choke the Iranian economy as dissident protesters in the Islamic Republic continue to rail against the hardline regime for its spending on foreign interventionism in hotspots such as Syria and its continued support for international terror groups.

Diplomatic tensions over the new sanctions have heightened ahead of November, when the Trump administration expects to fully reinstate sanctions on Iran that could cripple its business ties to Europe and further strain the regime's cash reserves.

Iranian officials have responded to the sanctions with threats of violence, including a possible military blockade of critical shipping routes in the Persian Gulf that has stoked fears of a potential war in the region. Top Israeli officials declared on Thursday that they would respond militarily should Iran take such actions.

Senior U.S. officials leading the efforts to pressure European partners over their business ties with Iran told the Free Beacon the Trump administration will not hesitate to sanction those who violate the new sanctions, warning that international banking institutions and even top U.S. banks could be hit with sanctions for not complying.

"The Iranian regime has and continues to use terrorism as a weapon in Europe," Richard Grenell, the U.S. ambassador to Germany, told the Free Beacon. "We must be vigilant in finding out about their plans and stopping them before they succeed."

Grenell and other top U.S. diplomats "are urging our partners to help stop the flow of money to this Iranian regime because it is used to fund malign activities," the ambassador said.

Administration allies in Congress have been helping to bolster these efforts.

Ten U.S. senators recently wrote to the EU3—Britain, Germany, and France—warning them to comply with all new U.S. sanctions coming down the pike.

As these European countries work to preserve the nuclear deal and keep business with Iran open, the senators ensured these countries they will not be kept safe from new U.S. sanctions.

"We write to urge you to comply with all American sanctions, but also to emphasize we would consider it particularly troubling if you sought to evade or undermine American statutes," write the 10 Republican senators, including Sen. Ted Cruz (R., Texas), Marco Rubio (R., Fla.), Tom Cotton (R., Ark.), and David Perdue (R., Ga.), among others.

"First, these statutes align with your governments' commitment to deepen cooperation addressing Iran's ballistic missile program and destabilizing activities," the letter states. "Second, they were passed over several years by overwhelming bipartisan majorities—including in some cases via unanimous Senate passage—and Congress is obligated to ensure their implementation."

"Any attempt to evade or undermine them could well prompt Congressional action, in coordination with other elements of the U.S. government, to ensure their integrity," the letter continues.

Senior congressional sources working on the matter who spoke to the Free Beacon said the decision is clear: Europe must choose between the United States and Iran.

"Everyone has gone out of their way to be nice to the Europeans while they've been working through the stages of grief over the Iran deal," said one senior congressional official who has been working on the matter. "There's deep appreciation across Congress for the importance of NATO and the other elements of the transatlantic relationship."

"But enough is enough," said the source, who would speak only on background when discussing efforts to pressure Europe. "Iran is threatening to shut down Gulf energy exports, they're promoting sectarian warfare across the Middle East, and they're launching terrorism globally and even in Europe. It's inexplicable and unacceptable for the Europeans to choose Iran over us."

A Treasury Department official, responding to queries from the Free Beacon on upcoming sanctions actions, said the administration could not specifically comment on potential actions, but pointed to wide-ranging sanctions guidance recently issued by the administration.

A full range of sanctions on Iran are expected to be put back in place by Nov. 4, with a range of new actions expected early this month.

The new sanctions will not only hit Iran and its global terror operations, but also international financial systems such as the Society for Worldwide Interbank Financial Telecommunication, or SWIFT, the major provider of global financial services.

American and European leaders on SWIFT are being told that they, too, will not be spared from new sanctions if they continue to facilitate business with Iran.

"There's no question the banks represented on the board of SWIFT are in serious jeopardy—both the American banks and the foreign ones," said Richard Goldberg, a senior adviser to the Foundation for Defense of Democracies and former senior Senate aide who was one of the architects of the initial SWIFT sanctions in early 2012.

"Congress authorized the president to take a wide range of actions against directors personally and against their financial institution employers," Goldberg explained. "The Trump administration understands how crucial it is to sever Iran from SWIFT so you can bet they will seek maximum enforcement of the sanctions."

Diplomatic warnings to SWIFT members have already been issued, according to sources familiar with ongoing actions to lay the groundwork for new sanctions.

"Let's just say that if I was a director or senior employee at SWIFT, I'd make sure I had all my money accounted for and didn't plan any vacations to the United States for a while," said one veteran foreign policy adviser familiar with the Trump administration's thinking on the issue.

"For the two American banks—Citi and JP Morgan—well, I wouldn't want to be in their C-Suites when Squawk Box is wall-to-wall covering the massive government action that just came their way for non-compliance," the source warned.