Seeking to reassure Washington's longtime ally Japan, Hagel's remarks and promise of more missile defence ships came as Tokyo faces a tense row with Beijing over islands in the East China Sea.
"All nations deserve respect, no matter how large or how small," Hagel said during a visit to Tokyo.
His comments also underscored a tougher line by the US government on China's approach to territorial claims in the South China Sea and the East China Sea, after some Southeast Asian countries accused Beijing of intimidatory tactics.
As "a great power," China has "great responsibilities," Hagel said.
A topic Hagel plans to raise with the Chinese this week is "respect for their neighbours," he said.
"Coercion, intimidation is a very deadly thing. It leads only to conflict," he said.
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