SINGAPORE - U.S. Defense Secretary Jim Mattis on Saturday slammed China's militarization of disputed South China Sea islands, insisting that weapons systems recently deployed in the area were meant to intimidate and coerce Beijing's neighbors.

The comments came during a speech at the Shangri-La Dialogue, an annual Asian defense forum in Singapore. In the speech, Mattis laid out the broader U.S. strategy for a "free and open" Indo-Pacific region.

"China's policy in the South China Sea stands in stark contrast to the openness our strategy promotes. It calls into question China's broader goals," Mattis said.

Specifically, the Pentagon chief mentioned China's deployment of anti-ship and surface-to-air missiles, electronic jammers, and the landing of a bomber aircraft at the Paracel Islands off the coast of Vietnam.

"Despite China's claim to the contrary, the placement of these weapon systems is tied directly to military use for the purposes of intimidation and coercion," Mattis added.

Though much of the world is focused on an upcoming summit between North Korea and the United States, this year's Shangri-La Dialogue has focused primarily on the region's long-term future and how to deal with a more assertive China.

Beijing has begun projecting power beyond its borders, most notably through the construction and militarization of islands in disputed areas of the South China Sea.

China claims nearly the entire South China Sea, despite overlapping claims by countries including Vietnam, the Philippines, Brunei, Malaysia and Taiwan.

Satellite imagery shows the deployment of several
FILE - Satellite imagery shows the deployment of several new weapons systems, including a J-11 combat aircraft, at China’s base on Woody Island in the Paracels, South China Sea, May 12, 2018.

Last week, the United States disinvited China from the Rim of the Pacific exercise (RIMPAC), a major international maritime exercise to be held later this year, citing Beijing's behavior in the South China Sea.

Mattis on Saturday referred to that disinvitation as an "initial response" to China, but he did not outline any additional steps that might be taken.

"The U.S. will continue to pursue a constructive, results-oriented relationship with China, cooperating when possible and competing vigorously where we must," Mattis said.

China blamed for tensions

During a question-and-answer session afterward, Mattis fielded a question from a Chinese colonel, who claimed that U.S. freedom of navigation operations are a provocation that violate international law. 

"I understand the disagreement, but it is not one on which we are unstudied," Mattis shot back, noting that the operations take place in international waters. "This is not a revisionist view."

Mattis laid the blame for rising South China Sea tensions on Beijing, saying it had ignored neighbors' concerns and an international tribunal's ruling on the issue.

"Nobody is ready to invade those features," Mattis said. "Certainly, we could have had the dispute resolution go in a peaceful way."

Australia's Defense Minister Marise Payne, from l
Australia's Defense Minister Marise Payne, from left, China's PLA Lieutenant-General He Lei and Canada's Defense Minister Harjit Sajjan listens to U.S. Secretary of Defense Jim Mattis' address at the Shangri-la Dialogue in Singapore, June 2, 2018.

China's reaction

Without mentioning Mattis, the head of the Chinese delegation denounced "irresponsible comments" that were made at the forum about China's military buildup in the South China Sea.

"Any irresponsible comments from other countries cannot be accepted," said Lieutenant General He Lei, vice president of China's Academy of Military Science. "It's within China's sovereignty to deploy troops and weapons on islands and reefs in the South China Sea. And it's allowed by international law."

Lei said the purpose of China's buildup was "avoiding being invaded by others" and contended that tensions in the region had eased significantly because of the joint efforts of China and the Association of Southeast Asian Nations. He also accused the U.S. of fueling tensions in the area.

"The purpose for the United States to mess up the situation at this moment is obvious," he said. "I think it is escalating the tensions to find more excuses for it to seek more military presence or actions."

President Donald Trump talks with Kim Yong Chol, f
President Donald Trump talks with Kim Yong Chol, former North Korean military intelligence chief and one of leader Kim Jong Un's closest aides, as they walk from their meeting in the Oval Office of the White House in Washington, June 1, 2018.

North Korea

The Shangri-La Dialogue comes as the United States and North Korea prepare to hold denuclearization talks in Singapore. President Donald Trump on Friday said the summit would take place June 12.

"Our objective remains the complete, verifiable and irreversible denuclearization of the Korean Peninsula," Mattis said.

The United States has long demanded that North Korea give up its nuclear weapons. But what's less clear is what Washington is prepared to offer Pyongyang. 

Mattis said any discussion about the future of 28,000 U.S. troops stationed in South Korea "will be separate and distinct from the negotiations" with North Korea.

"That issue will not come up in the negotiations with DPRK ... nor should it," he said, using an initialism for North Korea's official name. 

Instead, Mattis said any discussions about U.S. troops would be conducted between Seoul and Washington.

During his trip to Singapore, Mattis has been reluctant to discuss the summit preparations, instead insisting that the Pentagon is focused on backing up U.S. diplomats leading the process.

"The hopes of the world are on these talks," Mattis said.

U.S. Secretary of Defense Jim Mattis arrives to gi
U.S. Secretary of Defense Jim Mattis arrives to give an address at the Shangri-la Dialogue in Singapore, June 2, 2018.

But Japan's defense minister, Itsunori Onodera, warned that the negotiations in themselves were not the goal.

"I believe that it is important not to reward North Korea solely for agreeing to have a dialogue," Onodera said, noting that Pyongyang has made denuclearization promises in the past, only to backtrack.

During his speech, Onodera repeatedly spoke of the need to deal with the threat of North Korea's ballistic missiles "of all flight ranges."

A report in The Washington Post suggested that Japanese officials were concerned the United States may strike a deal with North Korea that removes the threat of intercontinental ballistic missiles, but not medium- and short-range missiles.

"These things fly over the Sea of Japan. They even fly over northern Japan. So that's where the security threat comes from for them," said Mintaro Oba, a former U.S. diplomat focused on Northeast Asia.

Oba said it would be important for the United States to closely consult with its allies in South Korea and Japan on its negotiations with the North.

"I think the most important thing for all of our allies is to stay relevant in the process and to demonstrate to their public that they've been closely consulted by the United States on this," he said.