China's "powerful economic engine is also funding a military modernization program that has raised concerns in the region — a concern also shared by the U.S. Pacific Command," Adm. Willard said.

He said China's announced interest in supporting a peaceful and stable political environment to support Beijing's overall development "is difficult to reconcile with new military capabilities that appear designed to challenge U.S. freedom of action in the region and, if necessary, enforce China's influence over its neighbors — including our regional allies and partners."

The comments were unusually candid for a senior combatant commander. In the past, most U.S. military commanders sought to emphasize that China's arms buildup did not pose a threat.

According to the Pentagon's report on China's military, Chinese military forces have been developing an array of advanced weaponry, including new nuclear ballistic and cruise missiles, anti-satellite weaponry and cyber-attack capabilities, in addition to new more conventional ships, aircraft and ground-warfare capabilities.

Chinese civilian and military leaders have said repeatedly in public statements that the buildup is defensive and poses no threat.

Adm. Willard said China's military buildup is "aggressive," and he disputed Chinese leaders' claims that the modernization is defensive.

"While PRC leaders have consistently characterized such developments as defensive in nature, the scope of modernization, the extensive commitment to advanced training, the development of robust power projection capabilities and, most importantly, Beijing's lack of transparency call such assertions into question," he said.

PRC stands for China's official name, People's Republic of China.

Adm. Willard also said China's rhetoric does not match its deeds. For example, he noted that contrary to assertions that China opposes the weaponization of space, it is building an anti-satellite capability that was tested in January 2007 when a weather satellites was shot down with a direct ascent missile.

"The PRC's stated goals of a defense-oriented military capability contributing to a 'peaceful and harmonious' Asia appear incompatible with the extent of sophisticated weaponry China produces today," he said.

The four-star admiral called for continuous frank conversations and mutual acton as part of military-to-military relations as the only way to reconcile the contradictions.

Adm. Willard said strong military relations between the Pentagon and China do not exist yet.

"Until it does and it is determined that China's intent is indeed benign, it is critical that we maintain the readiness of our postured forces; continually reinforce our commitment to our allies and partners in the region; and meet each challenge by the PRC in a professional manner that is consistent with international law," he said.

Last year, Chinese naval vessels harassed U.S. Navy ocean surveillance ships in the South China Sea and off the coast of northern China.

Tensions between the U.S. and China also were heightened in 2008 after the U.S. government announced a $6.5 billion arms package to Taiwan, which China views as its territory.