Coinbase says the S.E.C. has threatened to sue it over a plan to pay interest.

Coinbase, the largest cryptocurrency exchange in the United States, said on Wednesday that federal securities regulators were threatening to sue it over a proposed financial product that would let customers earn interest on digital asset deposits.

The company, in a regulatory filing, said the Securities and Exchange Commission notified it on Sept. 1 that its Lend product could violate securities laws. Regulators, the company said, might respond to Lend’s release by seeking a civil injunction.

The issue raised by Lend — an interest-generating service that somewhat resembles accounts traditionally offered by banks — is whether it will be engaged in trading or offering products to consumers that are considered securities, which the S.E.C. has the power to regulate.

The warning to Coinbase, which listed on the public market in April, is an indication that the S.E.C. is closely watching cryptocurrency companies — especially as they move into the territory of heavily regulated industries, such as banking. Gary Gensler, the S.E.C. chair, has said he is worried about the effects that unregulated crypto exchanges and products could have on the markets and investors.

Lend, which Coinbase announced in June, would allow customers to earn interest on cryptocurrency deposits. Specifically, customers would be able to earn interest on USD Coin, a so-called stablecoin whose value is tied to the dollar. Yields would be higher than those offered on classic bank accounts, and Coinbase would be among numerous cryptocurrency businesses entering this sector.

Coinbase executives pushed back against the S.E.C. in online postings, saying that the Lend program doesn’t qualify as a security and that the commission’s notice had caught them off guard.

“The S.E.C. has repeatedly asked our industry to ‘talk to us, come in.’ We did that here,” Coinbase’s chief legal officer, Paul Grewal, said in a blog post. “But today all we know is that we can either keep Lend off the market indefinitely without knowing why or we can be sued.”

Coinbase’s chief executive, Brian Armstrong, called the S.E.C. “sketchy” in an extensive thread on Twitter and said that he went to Washington in May to meet with financial regulators at many agencies. “The S.E.C. was the only regulator that refused to meet with me,” he said.

By seeking permission to act, Mr. Armstrong said, Coinbase was facing more resistance from regulators than other cryptocurrency companies that have launched similar products.

Andrew Calamari, a lawyer with Finn Dixon & Herling and a former director of the Securities and Exchange Commission’s New York office, said it appeared that the S.E.C.’s enforcement staff “thinks the offering would be illegal” if it went forward.

Some legal experts said securities regulators appeared to be taking a somewhat cautious approach in giving Coinbase a fair warning of its thoughts as opposed to simply letting the company go forward with the lending product and then suing them later.

Tyler Gellasch, a former S.E.C. official who leads the nonprofit Healthy Markets Association, said the commission recognized the importance of carefully handling a new kind of product entering the market. “This is a very large player in the cryptocurrency place and they are be extremely cautious in bringing down a hammer,” he said.

Shares of Coinbase were down about 4 percent on Wednesday morning.