Following months of groundwork, frenzied late-night phone calls and a potent pressure campaign from Ukrainian leaders, the U.S. and Europe banded together to impose what is shaping up to be the biggest coordinated package of sanctions ever levied against a major economy.

Over the past three days, that coalition of countries, representing some of the world’s biggest democracies, hit Russia with a series of increasingly severe economic penalties over its invasion of Ukraine, from direct sanctions on Russian President Vladimir Putin and restrictions on Russia’s central bank to a plan to block some financial institutions from Swift, an international payments system.

On Sunday, European Union officials set out more restrictions against Russia, including banning Russian airlines from the bloc’s airspace and extending sanctions to neighboring Belarus, a key staging point for the Ukraine invasion. U.S. and European officials are weighing additional moves if Mr. Putin doesn’t back down.

Financial markets were braced for a selloff in the ruble and other Russian-related assets as soon as Monday.

Financial pressure on Russia increased on other fronts. BP PLC said on Sunday it would part with its nearly 20% stake in Russian oil producer Rosneft, after facing pressure last week from the U.K. government. Over the weekend, French authorities intercepted a cargo ship owned by a sanctioned Russian bank in French territorial waters and escorted it to port, where it remains in French custody.

The flurry of activity comes after an extraordinary few days in which Washington, Brussels and London dramatically shifted gears on how hard to hit Mr. Putin economically, according to officials and diplomats familiar with the discussions. Moving from a coordinated package of modest measures in the early days of the crisis, they accelerated to a more unified effort that backers say is, taken together, stronger than anything the West has thrown at one country, at one time.

The moves “are unprecedented, in terms of their scope against a major global economic power, their care in trying to avoid blowback against the sanctioners, and the cohesion of the international coalition doing the sanctioning,” said Richard Nephew, a former senior U.S. sanctions official, who played a role in designing sanctions against Iran from 2006 to 2012.

Officials said they are reviewing sanctions on Russian companies and Russian billionaires, believed to be important to Mr. Putin’s hold on power. Roman Abramovich, the Russian billionaire owner of Chelsea FC, a storied British soccer team, said on Saturday he would relinquish stewardship of the team. Mr. Abramovich hasn’t been targeted with any Western sanctions, but he is among the best known of a handful of Russian oligarchs with far-reaching international business interests.

“We are targeting oligarchs’ private jets, we’ll be targeting their properties, we’ll be targeting other possessions that they have,” Liz Truss, Britain’s foreign secretary, told Sky TV Sunday morning. “And there will be nowhere to hide.”

The measures partially unplug Russia from Swift, or the Society for Worldwide Interbank Financial Telecommunication, a global messaging system for financial transactions. Swift is one of the primary ways that money moves between Russia and the rest of the world.

The debate over whether or not to disconnect Russia from Swift took on symbolic significance when Ukrainian President Volodymyr Zelensky publicly turned it into a measure of the West’s resolve to oppose Mr. Putin. He personally lobbied European leaders to switch off Swift for Russian banks, according to people familiar with the conversations.

Suddenly, a wonky issue that rarely caught international attention was fodder for cable news. U.S. lawmakers of both parties latched onto the issue, and protesters outside the White House gates demanded that President Biden ban Russia from Swift. Diplomats said the public’s sustained focus on Swift sent a signal that they needed to take action.

Out of the spotlight, American and European officials agreed on a potentially more powerful lever: depriving Russia’s central bank of the ability to sell its prodigious foreign reserves to bolster its own currency. The U.S., Europe and Canada pledged Saturday to prevent the Bank of Russia from deploying its $630 billion stockpile “in ways that undermine the impact of our sanctions,” they said in a joint statement Saturday.

The move directly targets the war chest that Mr. Putin has built up in recent years to help insulate Russia’s economy from outside pressures. It has few precedents. Officials said Sunday they were confident they could put the measure in place by Monday morning.

On Sunday, EU foreign policy chief Josep Borrell said the measures could affect around half the reserves the Russian central bank holds.

The move could be a blow to Russia’s financial system, limiting the government’s ability to defend the ruble in currency markets, to make overseas purchases and to backstop banks that have been targeted by international sanctions, economists and central-bank officials said.

“Symbolically speaking, it’s a nuclear bomb in the world of global finance,” said Sony Kapoor, chief executive of the Nordic Institute for Finance, Technology, and Sustainability, an Oslo-based think tank.

U.S. officials said they have long been eyeing Russia’s reserves. “That’s an impressive war chest if you can use it—so let’s make it useless,” a U.S. official said. “People have put a lot of emphasis on the idea that Russia was sanctions-proof. We always thought that was a myth.”

The wild card in targeting the central bank is that Russia might already have drawn down a substantial amount of its European reserves in recent months, according to people familiar with the matter. Roughly 22% of Russia’s international reserves were held in France and Germany in June last year, according to Russian central bank data. A French official believes those numbers have changed significantly since then.

Western powers’ decision to disconnect Russia from Swift will make it more difficult for Western companies to pay staff or buy goods from Russian counterparts. Some critics worried it might drive Russia closer to China’s own payments system, threatening to weaken a global financial system dominated by the U.S. dollar.