As Japan steps up to counter China's growing influence in Asia Pacific, Tokyo's efforts may not be as effective without "commitment" from the United States — a situation which may disadvantage regional countries as well as Washington, an expert told CNBC. Tokyo's efforts to counter Beijing's influence in Asia have to be balanced with the U.S. asserting its economic commitment to the region, said Stephen Nagy, senior associate professor in the department of politics and international studies at the International Christian University in Tokyo. "Japan can't just do this alone. It just doesn't have the resources ... It needs the economic (support) as well as the security that the United States can provide to the region," Nagy told CNBC on Tuesday. Japan agreed to deepen cooperation with Southeast Asia during the 10th Mekong-Japan summit held in Tokyo on Tuesday. The meeting is part of Japan's efforts to boost its presence in the region as China's political and economic clout grows through infrastructure projects such as the Belt and Road Initiative (BRI).

'Future of the world's economy'

The U.S. and its allies, including Japan, have adopted a "free and open Indo-Pacific" strategy to manage Beijing's aggressive expansion in the South China Sea. The Indo-Pacific area is of great strategic importance as it's the "future of the world's economy," Nagy said. The U.S. and Japan could lose out from capitalizing on Asia's rapidly growing consumer base if China dominates commerce and politics in the region, he added. Asian countries around the Indo-Pacific area will be looking at both the U.S. and Japan to counter rising Chinese influence, he said. "They would like a lot more of Japan but they would like the United States to not only have the military commitment to the region but also an economic commitment," Nagy added.

President Donald Trump sits in a fire truck while participating in a Made in America event, with companies from 50 states featuring their products, on the South Lawn of the White House in Washington on Monday, July 17, 2017. Andrew Harrer | Bloomberg | Getty Images

Cost of 'America first' policy

"It is unclear if President Trump and his supporters understand that without the United States within the region ... an America first policy may result in an America last policy," Nagy said. If Washington does not actively engage with countries in Asia Pacific, Nagy said, "my concern is that the region will become more dominated by geopolitics between China and other regional powers — that the consumer markets won't be open to American and other powers." In addition to losing access to commerce in the world's most populous region, there could be other destabilizing effects that may impact the U.S. in unpredictable ways, he warned. China's BRI comes with geopolitical ambitions of creating an economic hierarchy in the region — with Beijing at the top and other countries dependent on China, Nagy explained. This could make it difficult for those countries to stick to their independent political views, he added.