Why didn’t the U.S. cut off Russia from SWIFT? It’s complicated.

President Biden said on Thursday that the United States and Europe were united in their efforts to confront Russian aggression toward Ukraine with aggressive sanctions. However, there was one area where he suggested disagreement: SWIFT.

The Belgian messaging service, formally known as the Society for Worldwide Interbank Financial Telecommunications, connects more than 11,000 financial institutions around the world. It is viewed as a potential nuclear option in the world of sanctions because, if Russia was kicked off SWIFT, the nation would essentially be severed from much of the global financial system.

But doing so would not be simple and could come with its own set of costly complications for countries outside Russia, many of which are dependent on the country for energy, wheat and other commodities. That has made some nations skittish about pulling the trigger.

SWIFT is a global cooperative of financial institutions that began in 1973 when 239 banks from 15 countries got together to figure out how to best handle cross-border payments. It does not actually hold or transfer funds, but it allows banks and other financial companies to alert one another of transactions that are about to take place.

Blocking Russia from SWIFT would curb its ability to conduct international financial transactions by forcing importers, exporters and banks to find new ways to transmit payment instructions. Because of Europe’s heavy reliance on Russian energy exports, analysts said, there is a reluctance among some euro area leaders to take that step and risk those purchases by making doing business with Russia more costly and complicated.

The Financial Times reported on Thursday that Prime Minister Boris Johnson of Britain was pushing hard for Russia to be removed from SWIFT, while Chancellor Olaf Scholz of Germany said such a move should not be included in a European Union sanctions package.

Mr. Biden made the case on Thursday that the sanctions the United States imposed on Russian financial institutions would be as consequential as excising Russia from SWIFT. He said kicking Russia off the platform remains “an option” but that most of Europe opposes such a move for now.

“It is always an option,” Mr. Biden said. “But right now, that’s not the position that the rest of Europe wishes to take.”

The United States and Europe disagreed on whether to oust a country from SWIFT before, most recently in 2018, when the Trump administration wanted to cut Iran’s access. Ultimately, SWIFT cut ties to Iranian banks out of fear of being in violation of sanctions against that country.

Still, sanctions experts said that SWIFT was often overhyped as a tool and that cutting access could actually backfire by forcing Russia to find alternate ways to participate in the global economy, including forging stronger ties with China or developing a digital currency.

Emily Kilcrease, a senior fellow at the Center for a New American Security, argued that such an action could accelerate Russia’s efforts to expand the use of its own financial messaging service and drive it closer to China.

“There’s also this longer term question about whether de-SWIFTing in and of itself is just creating a lot of bad incentives for Russia,” Ms. Kilcrease said.

Michael Parker, a partner at the law firm Ferrari & Associates, suggested that blocking Russia from SWIFT would probably open the door to other workarounds, including finding alternative communications systems. A more effective first step, he said, would be to impose the type of bank sanctions Mr. Biden announced on Thursday.

“To actually cut Russia off from the U.S. banking system or the global banking system, the Russian banks would have to be sanctioned. And that’s what they did,” he said. “At the end of the day, this is a financial tool — hitting their major banks is about as far as we probably could reasonably go as far as a first line of sanctioning.”

Emily Flittercontributed reporting.