President Donald Trump has warned North Korea the United States is prepared to use the full range of its military power if needed to defend itself and its allies.

Key points: Donald Trump has signalled a willingness to negotiate with North Korea

Donald Trump has signalled a willingness to negotiate with North Korea South Korean President Moon Jae-in and Mr Trump agreed to apply maximised pressures and sanctions on Pyongyang

South Korean President Moon Jae-in and Mr Trump agreed to apply maximised pressures and sanctions on Pyongyang Hundreds of anti-Trump protesters rallied near the US Embassy, accusing the US President of raising tensions with North Korea

But in a notable shift from his aggressive rhetoric toward the hermit kingdom, Mr Trump has also signalled a willingness to negotiate — urging Pyongyang to "come to the table" and "make a deal".

Speaking at a joint news conference with South Korean President Moon Jae-in in Seoul, Mr Trump said while he hopes to use all tools short of military force, he is prepared to do whatever is necessary "to prevent the North Korean dictator from threatening lives … so needlessly".

"We cannot allow North Korea to threaten all that we have built," Mr Trump said on the first day of his two-day visit to South Korea.

"We hope to God we never have to use" the United States' full military capabilities, he added.

The US President is also calling for "worldwide action", and said "every responsible nation, including China and Russia" should push for an end to North Korea's nuclear weapons program.

Mr Trump said he had seen "a lot of progress" in dealing with North Korea, though he stopped short of saying whether he wanted direct diplomatic talks.

Mr Moon said he and Mr Trump agreed to apply maximised pressures and sanctions on North Korea until it returns to "sincere" talks on disarming its nuclear and missile programs.

Fears Trump's visit could aggravate tensions

Sorry, this video has expired Donald Trump and First Lady Melania are visiting South Korea as the second leg of their five-nation trip.

Landing earlier 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 visit that could aggravate tensions with North Korea.

South Korean police were on high alert to monitor protests by both critics and supporters of Mr Trump.

Surrounded by thousands of police officers and a tight perimeter created by buses, hundreds of anti-Trump protesters rallied at a boulevard near the US Embassy, holding banners that read "No Trump" and "No War".

The demonstrators accused the outspoken President of raising tensions with North Korea and pressuring Seoul to buy more US weapons.

They also criticised him for pressing Seoul to re-do a bilateral free trade deal between the countries so that it is more favourable to the US.

Sorry, this video has expired Dozens of anti-Trump protesters gathered in Seoul to protest against Mr Trump's visit to South Korea.

On Tuesday, Mr Trump flew by helicopter to Camp Humphreys, the largest US military base in the country, and met US and South Korean troops, along with Mr Moon.

Meeting with military commanders about the North Korea issue, Mr Trump said: "Ultimately it will all work out, it always works out, it has to work out." He did not elaborate.

Mr 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, Mr Moon told Mr 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 Mr Trump and North Korean leader Kim Jong-un have raised the stakes in the most critical international challenge of Mr 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.

Mr 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.

Reuters/AP