The U.S. Treasury Department on Wednesday accused three Mexican financial firms of laundering millions of dollars in connection with fentanyl trafficking by drug cartels, as the Trump administration presses ahead with its crackdown on the trade in illicit opioids.

The move, which prohibits some transfers of funds to the companies — Vector Casa de Bolsa, Intercam Banco and CIBanco — could raise tensions between the United States and Mexico. The Trump administration has already designated several Mexican cartels as foreign terrorist organizations. President Claudia Sheinbaum has also sought to appease President Trump by ramping up seizures of fentanyl in parts of Mexico and transferring dozens of cartel operatives to U.S. custody.

The accusations quickly raised eyebrows across Mexico since Vector is a large brokerage firm controlled by Alfonso Romo, a businessman who was chief of staff for Andrés Manuel López Obrador, the former Mexican president and founder of the country’s leftist governing party.

Vector rejected the accusations. “Our brokerage firm has operated under the highest standards of regulatory compliance, internal auditing and supervision,” the firm said in a statement. It added that the operations singled out by the Treasury “correspond to ordinary transactions with legally established companies.”

Mexico’s Finance Ministry, responding to the Treasury’s move, said in a statement that it had requested proof linking the financial firms with any illegal activities, but that “no evidence was received.”

The wire transfers mentioned by the U.S. Treasury, the ministry added, had been made to legal companies in China — transactions that it said “are carried out by the thousands” between Mexican and Chinese companies.