SINGAPORE - United States Acting Defence Secretary Patrick Shanahan urged allies and partners to invest in their sovereign future, as he reiterated America's commitment to the Indo-Pacific region, which he said remains its "priority theatre.

"We are where we belong. We are investing in the region. We are investing in you, and with you. And we need you to invest further in yourselves," he said in his opening remarks at the 18th Asia Security Summit of the Shangri-La Dialogue on Saturday (June 1) .

Countries in the region need to invest in their own defence to strengthen deterrence and build third-partner capacity to be able to play their part in upholding a rules-based international order and maintain a level-playing field, he added.

These investments should strengthen interoperability and allow for information to be shared with like-minded countries, and ensure that country networks are secure and trusted.

"You are buying a long-term relationship, not just a platform," he told the audience.

Mr Shanahan's address comes at a time of heightened tensions between the US and China on the trade front, and questions among policy makers and international observers on America's commitment to the Asia-Pacific. It also comes as China's influence has been growing and there has been discomfort over recent announcements and measures by the Trump administration on Indo-Pacific and the defence front.

Reiterating Washington's commitment to the region, the defence secretary said America's defence spending had gone up by 17 per cent in the past two years and it would increase its investments even further to keep up its engagement.

"No other nation can match the United States' ability to work across distance, cultures, languages, and time - and we are increasing the rate at which we do this," he told the audience of defence officials, observers and experts.

Countries in the region have benefited from this, he said, listing some of the recent engagements. For example, Japan hosts America's fleet and the USS Ronald Reagan, the only US forward-deployed aircraft carrier, as well as ballistic missile defence assets and more than 54,000 military personnel, among others.

This enables "unparalleled operational cooperation and lets us project strength together in support of regional stability," he said.

South Korea hosts 28,500 US service members, two fighter wings of F-16 and A-10 aircraft, and a host of other forces. With the Philippines, US has 200 bilateral defence activities planned for 2019 alone, while Thailand continues to host Exercise Cobra Gold - the largest multilateral exercise in the Indo-Pacific.

Mr Shanahan also said Washington is increasing its military ties with India, and Singapore remains a steadfast US partner.

"Singapore is our only major security cooperation partner in the region and provides valuable access to US navy ships and military aircraft whose presence contributes to security and stability in the region. And we are glad to have Singapore's fighter aircraft train alongside the US air force."

Still, there is a need to look beyond the present and envision possibilities for the future, said Mr Shanahan.

"The new age of threats and the technologies needed to meet them will allow for improved partnerships whose potential is beyond imagination," he told the audience.

The Shangri-La Dialogue is organised by the International Institute for Strategic Studies. The three-day discussions will conclude on Sunday.