An annual survey of foreign portfolio investments -- including U.S. stocks along with short-and long-term debt -- showed holdings rose by 8 percent, up from $17.1 trillion a year earlier, the department said in a statement on its website. Japan was largest investing country with $2 trillion, followed by the Cayman Islands at $1.7 trillion and the U.K. and China at about $1.5 trillion each. Luxembourg rounded out the top five at $1.4 trillion.

Foreign holdings of U.S. equities climbed to $7.2 trillion as of June 30, from $6.2 trillion a year earlier. Short-term debt holdings increased to $954 billion from $909 billion, while long-term debt holdings rose to $10.3 trillion from $10 trillion, the data showed.

Treasury Secretary Steven Mnuchin said in an interview last week that U.S. indebtedness to foreigners doesn't worry him. His comments follow a warning by the U.S. director of national intelligence, Dan Coats, that America's debt burden represents "a dire threat to our economic and national security."