WASHINGTON (Reuters) - U.S. authorities said on Wednesday it has obtained an asset freeze against 18 traders, who are primarily China-based, over an international market manipulation scheme that has reaped more than $31 million in illicit profits over several years.

FILE PHOTO: The U.S. Securities and Exchange Commission logo adorns an office door at the SEC headquarters in Washington, United States, June 24, 2011. REUTERS/Jonathan Ernst/File Photo

The U.S. Securities and Exchange Commission said the traders had been manipulating more than 3,000 U.S.-listed securities for at least six years, using multiple accounts to create artificially high or low prices to buy or sell at a profit.

The SEC said it believed the scheme, which created the appearance of trading interest in thinly traded securities, was ongoing.

“We allege that defendants engaged in an extensive manipulation scheme and went to great lengths to evade detection, placing trades in over one hundred separate accounts at several different brokerage firms and submitting falsified documents to open new accounts in the names of others,” Joseph G. Sansone, chief of the SEC’s Market Abuse unit, said in a statement.

In one instance, traders named Xiaosong Wang and Shun Sui allegedly earned about $6,233 in illegal profits by buying and selling 6,000 shares of Connecticut-based television and online programming producer Chicken Soup for the Soul Entertainment Inc over the course of two days in July 2018, authorities said.

The SEC detailed several similar schemes manipulating the prices of other securities, including consumer products company Helen of Troy Ltd, Ultragenyx Pharmaceutical Inc, and gold investor Global X Gold Explorers ETF.

“The defendants knew, or were reckless in not knowing, that they were engaging in a securities fraud scheme,” authorities said, adding that the traders received multiple warnings from brokerage firms about their manipulative trading.

The U.S. Attorney’s Office for the District of Massachusetts also announced criminal charges against two of the traders, Chinese citizens Xiaosong Wang and Jiali Wang. A public defender for the traders did not respond immediately to request for comment.

The two charged traders reside in China and have residences in Massachusetts.

It would be difficult for authorities to bring any China-based traders before U.S. courts, given Washington does not have an extradition treaty with Beijing.