US Defense Secretary Ashton Carter on Saturday warned that land reclamation efforts and a military buildup in the South China Sea could lead to conflict between nations in the region.

Speaking at a defense forum at the Ronald Reagan Presidential Library in California, the Pentagon chief also said America was adapting its military posture to counter increased Russian "aggression."

Appearing on the final leg of an eight-day trip that included meetings with defense ministers from several nations in the Asia-Pacific region, Carter said his concerns about the frantic pace of land reclamation in the South China Sea were broadly shared.

"The United States joins virtually everyone else in the region in being deeply concerned about the pace and scope of land reclamation in the South China Sea," Carter told an audience of senior defense figures.

Carter added he was worried about "the prospect of further militarization, as well as the potential for these activities to increase the risk of miscalculation or conflict among claimant states."

The Reagan National Defense Forum is an annual event that sees dozens of America's top defense figures -- including politicians from both political parties -- discuss America's defense policies.

Carter used his platform to take a swipe at recent Russian military moves.

"At sea, in the air, in space and in cyberspace, Russian actors have engaged in challenging activities," he said.

"And, most disturbing, Moscow's nuclear saber-rattling raises questions about Russian leaders' commitment to strategic stability, their respect for norms against the use of nuclear weapons and whether they respect the profound caution nuclear-age leaders showed with regard to the brandishing of nuclear weapons."

In an echo of some of Reagan's own attempts to use technology to counter a Soviet nuclear threat, Carter talked up some of America's new high-tech weaponry, including an electromagnetic railgun that can fire projectiles at an astonishing 4,500 miles (7,250 kilometers) per hour.

- 'Surprising' new technologies -

He added that the United States was modernizing its nuclear arsenal, investing in new technologies such as drones and a new long-range bomber, as well as lasers and new systems for electronic warfare.

The defense chief hinted at additional new weapons that would be "surprising ones I really can't describe here."

Additionally, "we're updating and advancing our operational plans for deterrence and defense given Russia's changed behavior," Carter said.

Russia annexed Crimea in 2014 and has been supporting a pro-Moscow separatist conflict in eastern Ukraine.

Russian jets also started bombing Syria on September 30.

Moscow says the campaign aims to counter the Islamic State militant group but the West says Moscow is trying to support President Bashar al-Assad's regime.

Carter's trip was dominated by questions over China's continued land reclamation efforts and military buildup in the South China Sea.

On Thursday, he flew out to the USS Theodore Roosevelt aircraft carrier as it was sailing in the South China Sea.

The enormous supercarrier was accompanied by the guided missile destroyer the USS Lassen, which last month sailed past a series of islets in Subi Reef in the Spratly Island chain.

There, China is using dredgers to turn reefs and low-lying features into larger land masses for runways and other military uses to bolster its claims of sovereignty in the region.

The Lassen conducted a "freedom of navigation operation" as a way to rebuff China's those claims.

"We've done them before, all over the world. And we will do them again," Carter said of the sail-by.

Carter said he chose to talk about Russia at the Reagan library as the Cold War was a defining theme of the US leader's presidency.

Still, he also made some conciliatory gestures to both China and Russia, suggesting there potentially was room for both countries to be part of broader international security structure.

"We do not seek a cold, let alone a hot war with Russia. We do not seek to make Russia an enemy," he said.

Gunboat diplomacy: US skipper recalls chummy exchanges with Chinese
Aboard The Uss Theodore Roosevelt (AFP) Nov 6, 2015 - As the United States and China engaged in a standoff last week over an American naval vessel in Chinese-claimed waters, frontline officers from the two sides bantered about chicken wings and Halloween preparations.

Washington sent the guided missile destroyer USS Lassen to the South China Sea for a "sail-by" of Chinese-built islands that have raised concerns over Beijing's deepening toe-hold in the strategic waterway.

The move brought angry denunciations from China, which said its sovereignty had been violated, with US defence officials countering that American ships would continue such sailings.

But despite the big-power ramifications, radio contact between the two sides at the scene was frequent and surprisingly relaxed, according to an account of the close encounter by the USS Lassen's skipper, Commander Robert Francis.

"Every day a US ship is down here, we interact with the Chinese," Francis said.

"It's not uncommon for one of my officers on the decks to pick up the radio and start talking (to the Chinese)."

Francis spoke to reporters on Thursday after being helicoptered over from his ship to the USS Theodore Roosevelt -- also cruising the South China Sea -- during a visit to the giant aircraft carrier by US Defence Secretary Ashton Carter, who was on a diplomatic swing through Asia.

"We picked up the phone and said, 'Hey, what are you guys doing this Saturday? We got pizza and wings, we're doing this, we're planning for Halloween as well,'" Francis said, recalling exchanges with the Chinese navy.

"So, discussions of that nature, just trying to show we are normal sailors like them, have families just like them."

- 'Just a normal day' -

On October 27, the USS Lassen cruised within about six miles (10 kilometres) of Subi Reef in the Spratly Islands, a sprinkling of reefs and islets at the heart of the South China Sea that also is claimed in whole or part by the Philippines, Vietnam, Malaysia and Taiwan.

Subi reef is just one of the South China Sea sites where Chinese dredgers are creating expanding land masses capable of hosting large facilities, including runways.

A Chinese destroyer shadowed the USS Lassen, repeatedly querying why it was in "Chinese waters".

"I wouldn't call them warnings," Francis said.

The US vessel's carefully stage-managed "freedom of navigation" transit -- close enough to see construction cranes and other features on land -- was meant to subtly challenge China's sovereignty claims and stress the right to international free passage.

The South China Sea has long been viewed as a potential flashpoint for military conflict.

Those concerns have grown in recent years as China has taken a more assertive stance toward its long-standing insistence that virtually the entire sea is sovereign Chinese territory.

The island-building programme has accelerated within the past two years, and rival claimants to the body of water say it violates a regional code against provocative moves that could upset the maritime status quo.

Francis said he was surprised by the attention drawn by last week's encounter, calling the episode "just a normal day."

"I got a call from my mother and she was going, 'Hey, what's going on with you in China? I heard you were in China,'" he said.

"I was like, 'Mom, I'm not in China. I'm OK, I'm on the ship.'"