Defense Secretary Jim Mattis this week voiced new U.S. opposition to China's continued militarization of islands in the South China Sea.

"We remain highly concerned with continued militarization of features in the South China Sea," Mattis told reporters on Monday as he traveled to Vietnam.

Mattis also said China is using predatory economics to seek control over other nations.

The Chinese are engaged in a global infrastructure development plan called the Belt and Road Initiative that U.S. officials have said is being used by Beijing to expand influence and control abroad, and expand Chinese military bases around the world.

Mattis said the predatory economic policies include loans "where massive debt is piled on countries that fiscal analysis would say they are going to have difficulty, at best, repaying in the smaller countries."

The defense secretary, echoing the new U.S. hardline policy toward China, said the United States is not seeking to "contain" China but wants more reciprocal relations.

"We seek a relationship with China that's grounded in fairness, reciprocity, and respect for sovereignty, and that means respect for international rules and for all nations' sovereignty, whether they're large or small," Mattis said.

The militarization of the disputed South China Sea islands includes deployment of advanced anti-ship and anti-aircraft missiles. Electronic jamming equipment also was fielded in recent months.

A senior Pentagon official disclosed in June that the missiles include YJ-12B anti-ship cruise missiles that are capable of targeting U.S. warships as far as 340 miles away.

The air defense missiles include either HQ-9A or HQ-9B long-range surface-to-air missiles with ranges of up to 184 miles.

"The missile systems are the most capable land-based weapons systems deployed by China in the South China Sea," a defense official said.

The former Marine Corps general said there will be periods of tensions when the United States and China "step on each other's toes."

"So we're going to have to find a way to productively manage our relationship," he said. "And the military relationship is to be a stabilizing force in the relations between the two countries."

China recently broke military ties with the Pentagon after the Trump administration imposed economic sanctions on the People's Liberation Army department in charge of arms procurement and the general who leads it.

The sanctions were a punitive reaction to China's purchase of Russian jets and missiles in violation of an American law designed to punish Moscow for its takeover of Ukraine's Crimea.

Mattis was originally scheduled to visit China as part of the Southeast Asia trip. The stop in Beijing was canceled after China informed the Pentagon that a Chinese official of equal rank to the defense secretary was unavailable. The move was a further reflection of Chinese military anger over the recent sanctions on the PLA.

China also canceled a port call by a U.S. warship to Hong Kong and put off U.S.-China military talks in Beijing. A Chinese admiral who was scheduled to meet with the Navy's top official, Chief of Naval Operations Adm. John Richardson was recalled as well.

In another sign of heightened tensions, the FBI also arrested a Chinese Ministry of State Security intelligence officer last week and charged him with economic espionage.

Tensions with China also increased after a Chinese warship recently passed within 45 yards of the guided-missile destroyer USS Decatur, nearly causing a collision.

Vice President Mike Pence, in a recent speech, also accused the Chinese of seeking to unseat President Trump in the 2020 elections.

Mattis denied the Pentagon is seeing a more confrontational military approach toward China.

"In the South China Sea, over many American administrations, we have said that in international airspace, international waters, we will fly or sail," he said.

"You've seen that continue. And my relationship with my counterpart [in China] has in no way been changed over this."

Regarding the Pence speech and a new approach to China, Mattis said the vice president's message is similar to his view that the United States and China will cooperate in some areas, like on pressuring North Korea within the United Nations.

On the South China Sea and other international waters China is seeking to control, "we continue to sail through areas," Mattis said, noting that Chinese President Xi Jinping promised at the White House several years ago that the Spratly Islands would not be militarized.

The militarization "happened, but our policy has not changed, we do not accept that," he said. "No one nation can change the international rules of the road."

"So I do not see this as any increase in military confrontation but a continuation of a longstanding American policy: International rule of law, prosperity for all nations, sail wherever they wish in international waters, and respect the sovereignty of all nations, peaceful resolution of disputes through international tribunals."

Mattis comments followed criticism from China's ambassador to the United States, Cui Tiankai, who on Sunday denied the administration's charges of political, military, and intelligence activities targeting the United States.

Cui asserted the Decatur was at fault in the South China Sea near-collision.

"So it’s at China’s doorstep. It's not Chinese warships that are going to the coast of California or to the Gulf of Mexico," Cui said on Fox News Sunday. "It's so close to the Chinese coast. So who is on the offensive? Who is on the defensive? This is very clear."

Cui also said U.S. arms sales to Taiwan, an independent country, were a violation of Chinese internal affairs.

Mattis disputed the ambassador's statement on the warship incident.

"If Chinese freighters or Chinese ships are in the Gulf of Mexico, that's international waters," he said. "The South China Sea is one of the most heavily trafficked international sea lanes of communication. So when Chinese ships are putting bumpers over the side of it, you don't do that when you're out in the middle of the ocean unless you're intending to run into something, and so I would just leave it at that."

Mattis said he expected regional states, such as Vietnam and other Southeast Asia nations, to raise their concerns about freedom of navigation in the South China Sea during meetings in Vietnam and later at a session of defense ministers of the Association of Southeast Asian Nations (ASEAN) in Singapore.

"They've all been very clear, over years, that all nations need to have freedom of navigation. All nations, not just ours," he said.

Mattis is making his second trip to Vietnam this year, a move that appears intended to garner support from Hanoi for stronger regional policies against Chinese encroachment.

He called Southeast Asia "kind of the geopolitical heart" of the region.

Mattis will meet Vietnamese Defense Minister Ngô Xuân Lịch in Ho Chi Minh City, formerly Saigon.

On Trump's recent comment that the defense secretary may be leaving the administration, Mattis said he has no plan to leave.

"I'm on his team," he said. "We have never talked about me leaving. And as you can see right here, we're on our way. We just continue doing our job."

Trump said during an interview on CBS "60 Minutes" that Mattis was "sort of a Democrat" and could be leaving.

Asked if he was a Democrat, Mattis said that since joining the Marines at age 18 he has been "proudly apolitical."

"Where am I today? I'm a member of the president's administration," he said. "And you have seen that President Trump's military policies, security policies, reaping significant bipartisan support. … my portfolio is bipartisan by its very basis, and that is the protection of the United States."

As for political affiliation, Mattis said, "I've never registered for any political party."