BEIJING (Reuters) - The U.S. and Chinese militaries have reduced the risk of encounters between them having "unintended consequences", a top U.S. official said on Monday, while China reiterated it would not accept interference in the South China Sea.

There have been a series of incidents in recent years, most in the disputed South China Sea, where the United States has accused Chinese military ships and aircraft of coming dangerously close to U.S. forces.

Visiting U.S. National Security Adviser Susan Rice said confidence-building measures had reduced risks and the United States valued progress in improving military-to-military ties.

"Our military leaders communicate more frequently and more directly than ever before in the past," Rice said in a meeting with a vice chairman of China's powerful Central Military Commission, Fan Changlong.

"While our forces operate in closer proximity to each other, the risk of unintended consequences has gone down thanks to the confidence-building measures that our two sides have put in place."

The United States and China have increasingly been at odds over China's claims to most of the South China Sea, a waterway through which $5 trillion of trade moves annually, where the United States has sought to assert its right to freedom of navigation.

China has stepped up its rhetoric in defense of its claims since an international court ruled this month that China did not have historic rights to the waters, raising concern that China would assert its position more forcefully.

There have been two close contacts between the two militaries since last month alone, with the U.S. accusing China of shadowing an aircraft carrier in the South China Sea and of unsafely intercepting a spy plane in the East China Sea, where China has competing territorial claims with Japan.

China said it was conducting routine operations in line with laws and rules.

Fan also emphasized the need to deepen military-to-military relations with the United States to "avoid misunderstanding and miscalculation".

But he also dismissed any notion that China would bow to pressure when it came to protecting its national sovereignty in the South China Sea.

"The Chinese people will not yield to outside pressure," he said, the defense ministry said in a statement after the meeting.

Fan said relations between their militaries faced "obstacles and challenges", which, if not properly handled, would "disturb and undermine" progress.

He said the deployment of an anti-missile system in South Korea would impact mutual trust. The United States is deploying the system in South Korea to protect it from North Korea.

(Reporting by Jake Spring; Editing by Ben Blanchard, Robert Birsel)