The U.S. military has slammed flights by Chinese warplanes in the disputed South China Sea last weekend that reportedly simulated a strike against the USS Theodore Roosevelt carrier strike group, but said that the exercise had posed no threat to the vessels.

Starting Jan. 23, the Chinese military flew a large number of warplanes, including nuclear-capable H-6K bombers and fighter jets into the southwest corner of Taiwan’s air defense identification zone (ADIZ). The timing coincided with the entry of the Roosevelt group into the South China Sea through the Bashi Channel, which separates Taiwan from the Philippines.

The Chinese warplanes’ flight may have been a response to the Roosevelt’s operations in the strategic waterway, some observers said, stoking concerns of the potential for a miscalculation erupting into conflict.