The sudden resignation of US Secretary of Defense Chuck Hagel will not have a negative impact on the warming US-China military relationship, experts said Monday.

Hagel's resignation Monday came as a big surprise in a difficult time as the US faces a number of critical security challenges, from the fight against the Islamic State in Iraq and Syria to a drawdown in Afghanistan.

"The US and Chinese presidents are deeply committed to making further progress, especially on avoiding accidents," said Bonnie Glaser, a senior adviser for Asia at the Center for Strategic and International Studies.

Chinese President Xi Jinping and US President Barack Obama stressed the importance of increasing military exchanges between the two countries during their meeting in Sunnyland, California, in June 2013, and in Beijing earlier this month.

Obama said he and Hagel had determined it was an "appropriate time for him to complete his service".

"We've modernized our alliances in the Asia-Pacific, updated our defense posture and recently agreed to improve communications between the US and Chinese militaries," Obama said.

Obama and Xi agreed at the Asia-Pacific Economic Cooperation meeting two weeks ago to develop a code of conduct for military encounters between the two countries' forces. The move is aimed at preventing dangerous confrontations and reducing tensions as the two militaries encounter each other more often both in the air and at sea.

"There is an agreement on the protocol for stronger and clear communication between the US military and Chinese military," White House press secretary Josh Earnest said. "Those kinds of agreements don't come by accident. That's an illustration of the kind of success of leadership that Secretary Hagel provided at the Department of Defense."

Hagel, 68, who became the 24th US defense secretary in February 2013, had privately expressed frustration to colleagues at the administration's strategy toward Iraq and Syria and at his lack of influence in the decision-making process, Reuters reported.

While some news reports said that Hagel, a decorated Vietnam War veteran, was forced to resign, Earnest said Obama and Hagel arrived at the decision together.

Hagel agreed to stay on until his successor is confirmed by the Senate, now controlled by the Republicans after the Nov 4 midterm elections.

Hagel became the first foreigner to visit China's first aircraft carrier, the Liaoning, when he visited China in April. His Chinese counterpart Chang Wanquan, the minister of defense, visited the US in August 2013.

Hagel has met many senior Chinese government and military leaders in the past 20 months. On Oct 20, he met with Chinese State Councilor Yang Jiechi in Washington to discuss issues pertaining to Obama's trip to China.

Charlie Dunlap Jr,a professor of the practice at Duke Law School and executive director of Duke University's Center on Law, Ethics and National Security, said Obama's move to replace Hagel makes sense.

"Mr Hagel was brought on board to facilitate the downsizing of defense, but world events are causing a rethinking of the wisdom of doing so," Dunlap said. "The skills required to address the range of today's very serious threats do not, in my view, play to Mr. Hagel's strengths.

"As to Hagel's replacement, I believe the president will look for a Democrat this time," Dunlap continued. "Bob Work, the highly regarded current deputy secretary, would have to be on the short list, but we also could see a woman like Michele Flournoy - who previously served as undersecretary of defense for policy - picked, as she has exceptional qualifications and is, I think, well respected by both parties."

Bilateral military ties have warmed up despite occasional frictions over time, such as US military surveillance along the Chinese coast, US arms sales to Taiwan, China's announcement of its East China Sea Air Defense Identification Zone and the tension over maritime territorial disputes over the South and East China seas that could potentially draw the US into armed conflict with China due to its treaty obligations to its allies.

In June, China's People's Liberation Army Navy for the first time participated in the Rim of the Pacific exercise, the world's largest naval war games, attended by 22 nations.

Speaking at the Washington Ideas Forum 2014 late last month, Hagel stressed the importance to get the US-China relationship right.

He said the two countries don't agree on everything. "But we should be focused; they should be focused. There are many ways where we can cooperate," he said.

"They are a great power," Hagel said. "They will continue to be a great power. We are a great power."

The defense chief reiterated that the US rebalance strategy to the Asia-Pacific region is not about trying to contain China or to cut China short.

"We don't want that to happen," he said.

Hagel also touted his long relationship with China dating back to 1983, when he visited the country for the first time as a businessman, and his later acquaintance with Chinese ambassadors during his service in the US Senate from 1997 to 2009.

Yuwei Zhang in New York contributed to this story.

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