The U.S. should strengthen its defense against growing threats from both China and Russia by focusing on Beijing as its main global competitor, according to the Joe Biden administration's national defense strategy.

Although Russia remains a acute threat to the United States under the strategy, it is completely deterred from attacking NATO, a senior defense official said.

China is a primary challenge to the United States because it is the only competitor with both the intent and increasing ability to systematically challenge the United States on all fronts, militarily, economically, technologically and diplomatically, a second senior Pentagon official said.

Senior Defense Department officials acknowledged heightened global tensions since the last national defense strategy was released in 2018, noting that the international security environment has deteriorated since then.

Defense Secretary Lloyd Austin said the U.S. is definitely concerned about the escalation in Ukraine.

He said Russia's use of nuclear weapons in Ukraine would lead to a very serious response from the international community.

China is the most comprehensive and serious threat to U.S. national security because of its coercive and increasingly aggressive efforts to reshape the Indo-Pacific region and international system to suit its interests and authoritarian preferences, the national defense strategy said.

China's rapid modernization of its armed forces and nuclear weapons is of serious concern to the United States. China probably intends to have at least 1,000 deliverable warheads by the end of the decade, the Nuclear Posture Review said.

The U.S. has seen the PRC become both more capable and more irresponsible in its behavior in the Indo-Pacific region, particularly in recent years, I would only point, as one example, to the deep modernization and diversification of the PRC's nuclear capabilities in recent years, the official added.

While China is a major challenge to the U.S., Russia remains a acute threat to the U.S., the documents said.

Russia's ongoing invasion of Ukraine underscores that the nuclear threat persists and could increase in an increasingly competitive and unstable geopolitical environment, the nuclear policy review said.

Russia continues to emphasize nuclear weapons in its strategy, modernize and expand its nuclear forces, and swing nuclear weapons in support of its revisionist security policy.

Russia plans to continue building up its nuclear arsenal, which currently includes up to 1,550 accountable deployable warheads on strategic carriers that are limited by the New START treaty, as well as nuclear forces that are not limited by any arms control treaty, the review said.