Russian firms’ efforts to make payments for goods in China as secondary sanctions fears spook local banks have generated a flourishing market for middlemen, four sources said, with up to half of transactions now handled by intermediaries.

The U.S. Treasury has warned it could apply sanctions on those facilitating transactions with Russia, calling on foreign banks to boost compliance.

Deterred by the threat, Chinese banks are limiting their dealings with Russian companies, which in turn are rushing to open accounts at the only Russian lender with a Chinese branch, causing a bottleneck at VTB Shanghai.