US President Donald Trump on Tuesday vowed to "figure it all out" with his South Korean counterpart Moon Jae-in, despite the two allies' differences on how best to address the DPRK's ongoing nuclear and missile programs.

Trump flew into Osan Air Base outside the capital Seoul, before taking a helicopter to the Camp Humphreys US military base where he was welcomed by Moon.

The leaders will hold talks later in the day at the presidential Blue House, followed by a welcome banquet.

Prior to his arrival, Trump tweeted "Getting ready to leave for South Korea and meetings with President Moon, a fine gentleman," adding "We will figure it all out!"

On Monday, Trump told Japanese Prime Minister Shinzo Abe that Washington has "all options on the table" regarding Pyongyang and said its nuclear ambitions are a "threat to the civilized world and international peace and stability".

Moon: No military action

As tensions over the DPRK's nuclear program soar, Trump has traded personal insults and threats of war with DPRK leader Kim Jong Un, but Seoul and its 10 million inhabitants would be on the front line of any conflict.

In previous tweets, Trump criticized Moon for "appeasement" as he favored engagement with Pyongyang to bring it back to the negotiation table.

Moon is mindful that much of Seoul is within range of the DPRK's artillery.

In an address to parliament last week, he demanded, "There should be no military action on the peninsula without our prior consent."

Anti-Trump protests in Seoul

Citizens' views have been mixed, with both "No Trump" and "Welcome President Trump" demonstrations taking place in downtown Seoul since the weekend, sometimes only a few blocks apart.

In an editorial on Tuesday ahead of Trump's arrival, the Korea Times said of the US president: "His 'rhetorical bombs' say it all."

It added: "However, calm Koreans are reported to be about the Trump-Kim war of words, we cherish our lives as much as Americans do theirs and feel scared by the outlook of war."

Kim Hyun-Wook, a professor at the Korea National Diplomatic Academy, said that the two allies have "subtle differences in their positions" and underlying suspicions about each other.

Trade

Another issue is likely to be trade, with Trump expected to bring up the renegotiation of the five-year-old US-Korea free trade agreement which he has called a "horrible deal" and a "job killer".

Trump has ordered a country-by-country, product-by-product review of the causes of the US' trade deficits with all nations and blamed its trading partners for enlarging trade deficits.

Ahead of his Asian tour, Trump said he will seek "free and fair" trade with these countries.