US Defense Secretary Lloyd Austin said on Monday Washington was closely watching China's development of advanced weapons systems, while declining to comment directly on a report that Beijing had tested a nuclear-capable hypersonic missile.
The Chinese foreign ministry denied the report in the Financial Times newspaper, which said Beijing had launched a rocket in July carrying a hypersonic glide weapon. Beijing said it was a space vehicle, not a missile.
The United States and Russia have both tested hypersonic weapons, usually defined as missiles which fly more than five times the speed of sound, in a race for a new generation of weapons that are harder to detect and block.
Austin told reporters during a visit to the former Soviet republic of Georgia: "We watch closely China's development of armament and advanced capabilities and systems that will only increase tensions in the region."
He said Washington would remain focused on the military challenge from Beijing.
Austin was speaking during a trip to Europe on which the Pentagon said he would "reassure allies and partners of America's commitment to their sovereignty in the face of Russian aggression".
He said the United States would partner with Georgia, which fought a losing war with Russia in 2008, to help it strengthen its defence and deterrent capabilities.