Esper has visited several regional US allies including Japan and Australia in his maiden tour abroad following his confirmation as Pentagon chief last month.

His diplomatic offensive comes as Washington seeks to reassure its traditional friends of its support in the face of China's more assertive foreign policy and military posture.

"The Chinese have a global approach to how they're trying to expand their interests and their tentacles around the globe, and we need to be able to compete with them in that space," Esper said.

The Pentagon chief alluded to Beijing's actions to reinforce its territorial ambitions in the South China Sea, which have alarmed neighbours with competing claims to the strategic waterway.

Washington stood for freedom of navigation in the waterway along with respect for national sovereignty, Esper said.

"China chooses to stand contrary to these values, preferring to coerce its neighbours into activities designed for Beijing's benefit."

The US and Mongolia have enjoyed strong ties for decades, and Washington sees the country as a strategic ally against its regional rivals Russia and China.

Mongolia depends on Russia for three-quarters of its oil and China for most of its trade, but sees US relations as a hedge against its neighbours.

Esper was invited to name a horse in a traditional gesture of respect for visiting state guests.

He chose the name Marshall, after the general and diplomat who drafted the US-funded plan for the reconstruction of Europe after World War II.