At his first summit with U.S. President Donald Trump on Friday, Japanese Prime Minister Shigeru Ishiba obtained reassurance regarding the close security ally's commitment but also received an indirect request for increased defense spending.

While no numerical target was mentioned, diplomacy and security experts said that if the United States makes an excessive demand in the future, it could end up weakening Japan and undermining the countries' alliance amid mounting military challenges posed by China and North Korea.

Japan should stand firm and reject unrealistic proposals by fully explaining to the United States its past and current efforts to bolster the alliance as it grapples with Trump's transactional approach, the experts added.

At a joint press conference after the summit with Ishiba in Washington, Trump, known for his "America First" agenda, said the United States remains "totally committed to the security of Japan," alleviating concerns about a potential reduction in U.S. engagement.

But the Republican president, after noting the growth of Japan's defense budget in recent years, pointedly added, "We look forward to seeing even more."

Ishiba responded that Japan will determine expenditures "on its own initiative."

Higher defense spending by Japan would help the United States to "mitigate security risks from China," said Yasuhiro Kawakami, director and senior research fellow at the Tokyo-based Sasakawa Peace Foundation.

Trump's doctrine of prioritizing U.S. interests made his push for Japan to expand its defense budget predictable during his second term that began Jan 20, as he is believed to be wary of China's state-of-the-art technologies, including in the area of artificial intelligence.

Japan had been bracing for a U.S. push on increased defense expenditure, as Trump has repeatedly urged NATO allies since his inauguration to allocate 5 percent of gross domestic product for military spending.

During his first presidency from 2017, Trump said NATO members should meet the goal of spending 2 percent of GDP on defense. Then U.S. Defense Secretary Mark Esper emphasized that the standard applied to "our allies and partners around the world."

Japan had long capped its defense outlays at about 1 percent of GDP but moved in late 2022 to gradually increase the budget to the 2 percent target by fiscal 2027.

In an alarming development for Japan, Trump named Elbridge Colby as undersecretary of defense for policy. The China hawk is known for advocating that Tokyo should immediately boost its defense budget to around 3 percent of GDP.

But Japan may not be able to finance defense spending equivalent to 2 percent of GDP, given that the nation's fiscal health is the worst among major industrialized countries, with public debt more than twice the size of its economy.

The minority government led by Ishiba's Liberal Democratic Party has not yet determined when to implement tax hikes to fund the higher defense spending. The ruling coalition of the LDP and its junior partner, the Komeito party, lost its majority in the House of Representatives election late last year.

Nobuhiko Tamaki, an associate professor at Chuo University in Tokyo who is well-versed in bilateral relations, said higher defense spending could limit Japan's investment in other areas, undermining economic security and its alliance with the United States.

A "reckless demand" from the United States, such as for 3 percent of GDP, could trigger a "fall of public support" for the alliance among Japanese citizens, Tamaki said, adding there would be no need to accept it.

As long as the United States continues to compete with China, Japan's "comprehensive strength," backed by social, political and economic stability will be essential for Trump, Tamaki said. Ishiba's government should not "brownnose or court" Trump.

Kawakami also said Trump is likely to ask Tokyo to contribute more to the costs of hosting U.S. troops in Japan, as the bilateral five-year agreement on the budget will expire in 2027. In the past, he criticized Japan for free riding on U.S. security guarantees.

Tokyo, however, covers more than 85 percent of the operating costs of the U.S. military in Japan, far higher than the around 30 to 40 percent shares shouldered by Germany, South Korea and other security allies, Kawakami said.

Moreover, costs for a committee tasked with reducing the burden on Okinawa Prefecture, which hosts many U.S. bases, and those related to the planned realignment of forces there mean Japan would pay 400 billion yen ($2.64 billion) annually, accounting for over 95 percent, he said.

"To deal with the Trump administration's deal-based diplomacy, Japan will be required to present specific bargaining chips and clearly outline the reality of its defense burdens," Kawakami said.