China, the top foreign holder, increased its holdings 1.7 percent to $1.2 trillion. Japan, the second largest holder, boosted its investment 0.2 percent to $1.12 trillion.
Demand kept rising in December even as Congress approached a deadline to raise the government's $16.4 trillion borrowing limit. In January, Congress approved a measure to temporarily suspend the borrowing limit until May 19. That has allowed the government to take on more debt while the debate continues.