China might be secretly conducting nuclear tests with very low explosive power despite Beijing’s assertions that it is strictly adhering to an international accord banning all nuclear tests, according to a new arms-control report to be made public by the State Department.

The coming report doesn’t present proof that China is violating its promise to uphold the agreement, but it cites an array of activities that “raise concerns” that Beijing might not be complying with the “zero-yield” nuclear-weapons testing ban.

The concerns stem from the high tempo of activity at China’s Lop Nur test site, extensive excavations at the site, and Beijing’s purported use of special chambers to contain explosions.

Another factor feeding U.S. suspicions is the interruption in past years of data transmissions from monitoring stations on Chinese territory that are designed to detect radioactive emissions and seismic tremors.