Despite U.S. President Donald Trump's reassurance that Japan and the United States are "very unified" on North Korea, concerns persist that Pyongyang may refuse to rid itself of medium-range missiles and that Washington might agree to wind back its military presence on the Korean Peninsula at the behest of the North's leader Kim Jong Un.

During two-day talks through Wednesday with Japanese Prime Minister Shinzo Abe in Palm Beach, Florida, Trump pledged to continue levying the "maximum pressure" of crippling sanctions on North Korea backed by the threat of military action until it gives up its nuclear weapons.

Trump also committed to push Kim to abandon not only intercontinental ballistic missiles that could strike the United States but also short- and medium-range missiles capable of hitting Japan and South Korea during their planned meeting to be held by early June.

Preparations for what will be the first U.S.-North Korea summit are underway, with Secretary of State-designate Mike Pompeo, currently director of the Central Intelligence Agency, meeting with Kim recently during a secret trip to North Korea.

Referring to Pompeo's meeting with Kim, Trump tweeted, "Denuclearization will be a great thing for World, but also for North Korea!" But some East Asian security experts say it may be difficult for Trump to persuade Kim to do away with medium-range missiles, even if he won a full denuclearization package.

"For the U.S. to insist on the elimination of missiles that can reach Japan will be difficult, as long as U.S. bases remain in Japan," said James Schoff, a senior fellow at the Carnegie Endowment for International Peace, a Washington think tank.

"From the North Korean perspective, they're vulnerable to U.S. bases and power projection."

Schoff said medium-range missiles are probably there to stay, but that Japan should place its top priority on helping achieve the permanent denuclearization of North Korea in collaboration with the United States, South Korea and other regional powers.

"Even if Japan is vulnerable to conventional missiles or even chemical weapon attacks by missiles, that's still much more manageable than a nuclear dynamic," he told Kyodo News.

An aide to the Japanese prime minister cautions Kim may demand that the United States suspend or scale down joint military exercises with South Korea -- the first step in a perceived attempt to weaken Washington's alliance with Seoul -- as part of a nuclear deal with Trump.

Such a scenario is precisely what China, a strategic rival of the United States, would like to see, said Katsuyuki Kawai, a Liberal Democratic Party lawmaker who is special adviser on foreign affairs to Abe.

(Kim Jong Un, Xi Jinping and their wives drink tea in Beijing on March 27)

Ahead of a Trump-Kim summit and a preceding meeting between Kim and South Korean President Moon Jae In, slated for April 27 at the truce village of Panmunjeom, Kim met with Chinese President Xi Jinping in late March in Beijing in his first known trip outside North Korea since assuming power in 2011.

Kim's ultimate goal, Kawai argues, is to see the withdrawal of 28,500 U.S. troops from South Korea and the breaking of Washington's alliance with Seoul, a development that would match China's interests as well because it would alter the military balance in East Asia.

"Driving a wedge into security coordination among Japan, the United States and South Korea is precisely what China has in mind," Kawai said during a visit to Washington last month for talks with U.S. lawmakers and experts.

"From the Chinese perspective, Kim is actually a 'dutiful little brother' of Xi because he willingly tries to resolve problems in favor of China."

Kawai said that in exchange for extending cooperation in resolving the North Korean nuclear issue, Xi may call for a phase-out of the U.S.-South Korea exercises which Pyongyang has long considered an invasion rehearsal. Xi may also request a reduction of U.S. forces in South Korea and eventually the complete withdrawal of the U.S. military from the country.

Christopher Hill, a former U.S. chief negotiator on North Korean nuclear issues, urged the Trump administration not to make any concessions that would suggest a weakening of the alliance or commitment to South Korea, such as a reduction of the frequency and schedule of annual joint military exercises.

"While North Korea may claim that the purpose of its nuclear program is to defend against security threats posed by the U.S., the real purpose of North Korea's nuclear arsenal is to cause the U.S. to decouple its security relationship from the Republic of Korea," Hill told a House Foreign Affairs Committee hearing on April 11, referring to South Korea by its formal name.

"President Trump should reaffirm our commitment to our ally and work closely with China and others in the region, especially our brave ally, Japan, so that North Korea does not miscalculate U.S. resolve, so that other allies in the region and around the world are reaffirmed in their confidence in the U.S., so that we are able to maneuver from a position of strength, and so that any solution is sustainable," he said.