Behind a set of imposing metal doors in an easy-to-miss office building in a New York City suburb, a small team manages billions of dollars for a Russian oligarch.

For years, a group of wealthy Russians have used Concord Management, a financial-advisory company in Tarrytown, N.Y., to secretly invest money in large U.S. hedge funds and private equity firms, according to people familiar with the matter.

A web of offshore shell companies makes it hard to know for sure whose money Concord manages. But several of the people said the bulk of the funds belonged to Roman Abramovich, a close ally of President Vladimir V. Putin of Russia.

Concord is part of a constellation of American and European advisers — including some of the world’s largest law firms — that have long helped Russian oligarchs navigate the Western financial, legal, political and media landscapes.

both said they were leaving Russia. A spokeswoman for another large firm, Debevoise & Plimpton, said it was terminating several client relationships and would not take any new clients in Moscow. Ashurst, a large London-based law firm, said it would not “act for any new or existing Russian clients, whether or not they are subject to sanctions.”

The accounting giants PwC, KPMG, Deloitte and EY — which have provided extensive services to oligarchs and their networks of offshore shell companies — also said they were leaving Russia or severing ties with their local affiliates.

wrote a letter to the White House arguing that Russia’s Sovcombank shouldn’t face sanctions, citing the bank’s commitment to gender equity, environmental and social responsibility.

Sovcombank had agreed to pay the lobbyist’s firm, Mercury Public Affairs, $90,000 a month for its work.

The Biden administration recently imposed sanctions on Sovcombank. Within hours of the announcement, Mercury filed paperwork with the Justice Department indicating that it was terminating its contract with Sovcombank.

As recently as mid-February, the British law firm Schillings represented the Russian oligarch Alisher Usmanov, a longtime ally of Mr. Putin.

Two weeks later, the European Union and the U.S. Treasury placed sanctions on Mr. Usmanov. Nigel Higgins, a spokesman for Schillings, said the firm is “not acting for any sanctioned individuals or entities.”

say on its website that it represents “some of Russia’s largest companies,” including Gazprom and VTB. The firm said it was “reviewing and adjusting our Russia-related operations and client work” to comply with sanctions.

In Washington, Erich Ferrari, a leading sanctions lawyer, is suing the Treasury on behalf of Mr. Deripaska, who is seeking to overturn sanctions imposed on him in 2018 that he claims have cost him billions of dollars and made him “radioactive” in international business circles.

And the lobbyist Robert Stryk said he had recently had conversations about representing several Russian oligarchs and companies currently under sanctions. He previously represented clients targeted by sanctions, including the administrations of President Nicolás Maduro of Venezuela and former President Joseph Kabila of the Democratic Republic of Congo.

Mr. Stryk said he would consider taking the work if the Treasury Department provided him with the necessary licenses, and if the prospective clients opposed Russia’s aggression in Ukraine.