Christine Wormuth, the US under secretary of defense for policy, speaks at a military strategy forum at the Center for Strategic and International Studies in Washington on Monday. Chen Weihua/China Daily

A senior Pentagon official would not answer specifically whether she agreed with naming Russia and China as the biggest existential threat to the United States or with a US State Department statement dismissing the characterization.

Christine Wormuth, US under secretary of defense for policy, only said that the US and China have aspects of their relationship that are very cooperative "and there are aspects of our relationship that are more competitive,"she told a military strategy forum at the Center for Strategic and International Studies in Washington on Monday.

"Our goal is very much to see China rise and join the international community in a positive way,"she said.

Wormuth cited the large Chinese delegation in town to attend the 7th China-US Security and Economic Dialogue (S&ED) late last month and also the Strategic Security Dialogue she attended. She called it a great opportunity to talk very frankly to various Chinese officials about security, about working together and also about some of the challenges they faced.

The S&ED has been regarded by both governments as a high-level platform to expand bilateral cooperation while managing differences between a rising power and the existing one.

Wormuth's words came after last Thursday's Senate confirmation hearing on Marine Gen Joseph Dunford's nomination to be the next chairman of the Joint Chiefs of Staff. Asked by lawmakers, Dunford named Russia and China as presenting the greatest threat to US national security.

On Friday, US State Department spokesman Mark Toner said Secretary of State John Kerry did not share the view. "The secretary doesn't agree with the assessment that Russia is an existential threat to the United States, nor China, quite frankly,"Toner told a daily news briefing.

"These are major powers with whom we engage and cooperate on a number of issues, despite any disagreements we may have with them,"he said. "Certainly we have disagreements with Russia and its activities within the region, but we don't view it as an existential threat."

White House spokesman Josh Earnest also said on Thursday that Dunford's comment reflected "his own view and doesn't necessarily reflect the... consensus analysis of the president's national security team."

On Monday, Wormuth cited the cooperative aspects of the US-China relationship, such as counter piracy and issues like Afghanistan, but also noted the challenges in such areas as the South China Sea, cyber security and outer space.

"There are challenges to manage with China,"she said.

While the military-to-military relationship between the two countries has long been regarded as the most underdeveloped dimension of the overall relationship, China and the US have stepped up their military exchanges and cooperation in the last few years, with more high-level visits and joint exercises.

Just before the S&ED last month, Fan Changlong, vice-chairman of China's Central Military Commission, visited the US amid growing tensions in the South China Sea and cyber security arena.

China's People's Liberation Army also participated in the US-led Rim of the Pacific naval war games for the first time in 2014.

On Monday, Wormuth described the US' relations with China and Russia as two very important relationships. She said the US is facing both a set of short-term challenges and some long-term challenges and does not have the luxury of focusing on just one set of problems.

"We have to walk and chew gum at the same time,"Wormuth.

Compared with her comments on China, Wormuth was much harsher on Russia. Describing Russia as a serious issue for the US, she said Russia has an extremely large nuclear arsenal, it has been modernizing its conventional military, it has been developing a number of asymmetrical capabilities and cited its behavior in Ukraine.

Wormuth also described the security environment the US is facing as incredibly difficult and perhaps the most challenging the US has faced in the last couple of decades.

Wormuth discussed the US strategy in Afghanistan and Iraq and against ISIL. "We cannot allow Iraq and Syria to become long-term safe havens for ISIL,"she said.

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