* Trump greeted by US troops in S.Korea
* Visit may aggravate tensions with N.Korea
* US carrier groups to conduct exercises in region
President Donald Trump on Tuesday flew into South Korea, the closest he has come to the frontlines of the nuclear standoff with North Korea, saying a solution must be found to the security threat posed by Pyongyang.
Landing at Osan Air Base outside Seoul, the president and First Lady Melania Trump stepped down from Air Force One onto a red carpet as he began a 24-hour visit that could aggravate tensions with North Korea.
He then flew by helicopter to Camp Humphreys, the largest US military base in the country, and met US and South Korean troops, along with South Korean President Moon Jae-in.
The White House says Trump's trip is intended to demonstrate US resolve over his hardline approach to the North Korean nuclear and missile threats, but many in the region fear further bellicose presidential rhetoric could increase the potential for a devastating military conflict on the Korean peninsula.
Meeting with military commanders about the North Korea issue, Trump told reporters: "Ultimately it will all work out, it always works out, it has to work out”. He did not elaborate.
Trump praised president Moon, hailing him for "great cooperation," despite differences over how to confront North Korea and over a trade pact between the United States and South Korea.
When the two leaders later held formal talks after an elaborate welcoming ceremony outside the presidential Blue House in Seoul, Moon told Trump he hoped his visit would relieve some of South Koreans' anxiety over North Korea and serve as a "turning point in resolving the North Korean nuclear issue".
Pyongyang’s recent nuclear and missile tests in defiance of UN resolutions and an exchange of insults between Trump and North Korean leader Kim Jong Un have raised the stakes in the most critical international challenge of Trump’s presidency.
Three US aircraft carrier strike groups will exercise together in the Western Pacific in the coming days in a show of force rarely seen in the region, US officials said.
On the second leg of his five-nation trip, Trump toured the sprawling Camp Humphreys garrison, which lies about 100 km from the border with reclusive North Korea.
Trump was greeted with applause and a few cheers as he and president Moon entered the mess hall at lunch hour.
Trump is seeking to ratchet up pressure on Pyongyang following his visit to Tokyo, where he declared that Japan would shoot North Korean missiles "out of the sky” if it bought the US weaponry needed to do so, suggesting the Japanese government take a stance it has avoided until now.
N.Korea quiet about Trump visit
North Korea has not conducted a missile test for 53 days, the longest such lull in testing this year. North Korean state media has not commented on Trump's arrival in the South.
South Korea’s spy agency said last week that North Korea may be preparing another missile test, raising speculation that such a launch could be timed for Trump’s trip to the region.
US officials have said privately that intercepting a test missile is among options under consideration, though there is disagreement within the administration about the risks.
The South Korea leg of Trump’s trip is an effort to present a united front despite differences with Moon over how to confront North Korea, as well as Trump's complaints over the two countries' trade agreement and South Korean defence spending.
Trump has criticised Moon over his support for diplomatic engagement with Pyongyang - something the US president once called "appeasement” - and has threatened to pull out of a free trade pact between the two countries.
"Hopefully that will start working out, and working out so that we create a lot of jobs in the United States which is one of the reasons that I'm here," Trump told reporters, referring to trade issues between the two countries.
Several hundred supporters and protesters lined the streets of downtown Seoul as Trump’s motorcade passed by, waving flags and posters, with some saying, "No Trump, No War, Yes Peace,” while others cheered, "Trump! Trump!”
Trump will deliver a speech on Wednesday to South Korea's National Assembly expected to focus heavily on his North Korea policy, which has stressed sanctions and military pressure instead of diplomatic engagement with Pyongyang.
Trump has rattled some US allies with his vow to "totally destroy” North Korea if it threatens the United States, for deriding Kim as a "Rocket Man on a suicide mission” and for dismissing as pointless any talks with Pyongyang.
Kim, who has also made clear he has little interest in negotiations, at least until North Korea develops a nuclear-tipped missile capable of hitting the United States, has called Trump a "mentally deranged US dotard.”
The North accuses the United States, which has 28,500 troops in South Korea, a legacy of the 1950-53 Korean war, of planning to invade and regularly threatens to destroy it and its Asian allies.
Trump's visit to Camp Humphreys gave him a first-hand view of the massive military assets the United States has in place in South Korea, but it also could serve as a reminder of the cost in US military lives - as well as the potential massive South Korean civilian losses - if the current crisis spirals into war.
Trump, however, will steer clear of the heavily fortified demilitarised zone (DMZ) zone on the border between the two Koreas - where other US presidents have visited - a move that might have been seen by North Korea as more of a provocation.