US regulators have become increasingly concerned in recent months about unstable trading in the $12.6 trillion US Treasury market, where investors turn for safe-haven securities and set interest rates that are a benchmark for much of the rest of the global financial system.

Investors have complained that trading in large blocks of treasuries has become increasingly challenging in recent years and yields appear prone to occasional lurches, developments that could become a problem if the Federal Reserve starts raising short-term interest rates later this year, as is expected.