WASHINGTON - The United States will not be intimidate and will continue to operate in the South China Sea despite Beijing's attempts to militarize the region , US Defence Secretary James Mattis said in an address at a security conference in Singapore on Friday.

"The United states will continue to fly, sail and operate wherever the international law allows and our national interests demand," Mattis said. "We will not be intimidated and we will not stand down, for we cannot accept the PRC's militarization of the South China Sea or any coercion in this region."

Mattis is currently visiting Singapore for the annual Association for Southeast Asian Nations (ASEAN) defence ministers’ meeting. On Friday, the defence secretary met with Japanese counterpart Takeshi Iwaya and South Korean Defence Minister Jeong Kyeong-doo.

Mattis pointed out the importance of the need for strong cooperation along with understanding that no nation can change the international rules.

The relations of a number of Asian states, including China, Japan, Vietnam and the Philippines, are tense with respect to claims in the South China See and the East China Sea as they cannot reach agreement on maritime borders.

In addition, China and the United States have become increasingly embroiled in a trade dispute that has seen the two sides impose tariffs on billions of dollars’ worth of goods.

Meanwhile, China and Southeast Asian states will hold their first joint maritime exercises next week, officials said Friday, in a move aimed at easing tensions but which may spark US alarm.

Beijing's expansive claims to the South China Sea have long been a source of friction with rival claimants in Southeast Asia, as well as Washington which has traditionally been the dominant naval power in the area.

Despite disagreements over Beijing's territorial ambitions, China and Southeast Asia are trying to strike a more conciliatory tone in an effort to stop tensions from spiralling dangerously out of control.

As part of this, the navies of China and the 10-member Association of Southeast Asian Nations (ASEAN) are set to hold their first joint drills, which will take place in the South China Sea.

"As we speak, the navies of ASEAN are en route to Zhanjiang in China for the ASEAN-China Maritime Exercise," Singapore Defence Minister Ng Eng Hen said.

Making the announcement at a gathering of ASEAN defence ministers in Singapore, also attended by US Defence Secretary Jim Mattis and his Chinese counterpart, Ng said the drills would help to "build trust, confidence".

The city of Zhanjiang in southern China is home to the South Sea Fleet of the People's Liberation Army.

Tabletop exercises between ASEAN and China were held in Singapore in August to prepare for next week's drills.

However, some observers see the exercise as part of efforts by China to diminish American influence in the region by forging closer ties with Washington's traditional allies and partners.

In an apparent effort to lessen any such fears, Ng said ASEAN was planning to hold maritime exercises with the US for the first time next year.

Mattis insisted that he did not believe the China-ASEAN drills would reduce US sway in the strategically vital region.

"We do not see this as contrary to our interests," he said. "If we can have that sort of activity going on, it's transparent, something that's been lacking in the South China Sea. Then that's going to be working in the right direction."

Hoang Thi Ha, a political analyst with the ISEAS Yusof Ishak Institute in Singapore, said next week's exercises were aimed at "demonstrating that China and ASEAN are managing well their maritime problems".

But she added its significance should not be overstated as it was a one-off drill in waters that were not disputed.

"The US is not losing ground exactly because ASEAN chooses to be open and inclusive," she told AFP.

The Southeast Asian defence ministers also agreed on guidelines to prevent unplanned encounters between their military aircraft, and will propose them to other countries, including the US and China, at a meeting on Saturday, Ng said.

They are aimed at reducing the likelihood that a chance encounter could spiral into conflict in the case of a miscalculation.

Four ASEAN members - Brunei, Malaysia, the Philippines and Vietnam - have conflicting claims in the South China Sea with Beijing. China claims sovereignty over almost the entire area, including waters near the shores of smaller countries.

Cambodia, Indonesia, Laos, Myanmar, Singapore and Thailand are ASEAN's other members.

Concerns have escalated dramatically in recent years as China started building artificial islands on reefs in disputed waters, and it has also constructed military facilities and airstrips.

Washington has expressed alarm over the island-building, saying it could affect freedom of navigation in the sea, which hosts some of the world's most vital commercial shipping lanes.

At Friday's meeting, Mattis stressed that "no single nation can rewrite the international rules of the road, and we expect all nations - large and small - to respect those rules".

The Pentagon chief met Thursday with his Chinese counterpart, General Wei Fenghe, on the sidelines of the gathering as they sought to normalise military relations that have dramatically soured over trade and sanctions tensions.