When South Korean President Lee Jae Myung meets President Donald Trump next week for their first summit, he will be asked to pay more for the upkeep of American troops on the peninsula during talks set to be dominated by security issues and China.

Left largely out of the frantic trade talks that culminated in an unwritten deal last month, questions over the future of the South Korea-U.S. alliance and the approach to nuclear-armed North Korea will be a key part of the White House discussions, officials and analysts said.

A thorny issue for Lee may be Trump's push for Seoul to pay significantly more for the 28,500 U.S. troops stationed in South Korea as a legacy of the 1950-1953 Korean War.

A U.S. official, speaking on condition of anonymity, said a key area of focus would be so-called burden-sharing, and Trump is expected to push the South Koreans for more.

Seoul provides more than $1 billion a year to support the U.S. troop presence and also paid to build the largest U.S. base overseas, Camp Humphreys, said Victor Cha, of Washington's Center for Strategic and International Studies.

"But President Trump clearly wants more," Cha said, noting his past calls for Seoul to pay $5 billion or even $10 billion. "He wants defense spending closer to 5% of GDP for all allies; South Korea is currently at 3.5%."

South Korea wants to modernize the U.S. alliance to adapt to the changing security environment such as U.S.-China rivalry and is looking at higher defense spending, Seoul's top security advisor Wi Sung-lac said.

"The issue (spending) is being discussed between South Korea and the United States, and the figures and such are still in progress and under discussion," Wi told a press briefing.

There are discussions within the Pentagon about removing some U.S. troops from South Korea, the U.S. official said.

To head that off, some at the Pentagon are trying to refocus the alliance towards the threat posed by China.

While a lot will depend on Seoul's willingness, the U.S. official said the desire was to discuss in broad terms how the alliance, and U.S. forces in South Korea, could be used to counter China.

That could bring more headaches for Lee, who has expressed full support for the U.S. alliance but vowed to take a balanced approach between Washington and Beijing.

General Xavier Brunson, commander of U.S. Forces Korea (USFK), said this month it was not a foregone conclusion that South Korea would be involved with any conflict over Taiwan, which China claims as its own.

However, he said there should be a recognition that nothing happens in the region in isolation and U.S. troops in South Korea could be needed to "solve bigger problems."

Asked about more such "strategic flexibility," South Korea's Foreign Ministry said it was difficult to predict USFK operations in a hypothetical situation.

"However, the operation of the USFK is carried out under close consultation and communication between South Korea and the United States," a spokesperson said.

Lee and Trump are likely to see eye to eye on North Korea, with both open to engaging its leader Kim Jong Un, and the U.S. president frequently casting himself as a global peacemaker.

However, Pyongyang has rebuffed attempts to revive the unprecedented diplomatic engagement seen in Trump's first term and doubled down on its deepening ties to Russia.

"I doubt much substance will be said on North Korea beyond committing to diplomacy and reiterating a goal of denuclearisation," said Jenny Town, of the Washington-based 38 North program, which monitors North Korea.

Lee told a Japanese newspaper on Thursday his administration would lay the groundwork to ultimately dismantle North Korea's nuclear weapons program, through talks with Pyongyang and close cooperation with Washington.

North Korea has repeatedly said its nuclear weapons are not open for negotiation.

South Korean Foreign Minister Cho Hyun told parliament this week South Korea could also try to use the summit to win approval to reprocess or enrich its own nuclear materials.

Despite talk from some South Korean officials about the need to attain "nuclear latency," or the means to quickly build an atomic arsenal, Cho said reprocessing would be only for industrial or environmental purposes.

Daryl Kimball, executive director of the U.S.-based Arms Control Association, said there was no practical "industrial or environmental" need for South Korea to start reprocessing, or for a domestic uranium enrichment capability for its energy program.

Both activities are prohibited under the current U.S.-South Korea Agreement for Nuclear Cooperation because they could be used to produce nuclear bombs, he said.

"Too many South Korean politicians are flirting with the idea of their country acquiring the nuclear weapons option," he added.