SEOUL (THE KOREA HERALD/ASIA NEWS NETWORK)- President Moon Jae-in's mission in upcoming summit talks with US President Donald Trump is clear: dispel scepticism about the South Korean government's position on relations with the US and its policy on North Korea.

Such scepticism is especially rampant when a liberal president takes power in South Korea while a Republican leader occupies the White House. This is partly due to how South Korean liberals tend to lean toward North Korea and distance themselves from the US.

Whether intended or not, Moon has fuelled the scepticism. He halted the deployment of a US missile shield system in South Korea, citing the need for a legitimate, transparent process, including an environmental review.

The South Korean president essentially put the brakes on an "alliance decision."

Moon also offered one peace overture after another toward the North at a time when the US and the international community were toughening sanctions against North Korea. He said he would be willing to hold a summit with North Korean leader Kim Jong-un. His administration also authorised aid programmes for the North and proposed inter-Korean sports exchange programmes.

Then his security adviser created a stir here and in the US by suggesting scaling down joint South Korea-US military exercises and cutting back the deployment of US strategic assets on the Korean Peninsula if North Korea freezes - not dismantles as sought by the US - its nuclear and missile programmes.

A joint letter by a bipartisan group of 18 US senators demonstrates how the US is taking the latest developments in South Korea. The letter was addressed to Trump, but what the senators emphasised may well be applied to Moon, too.

What the senators mentioned in the letter constitutes the most contentious issues Moon and Trump should tackle in their first summit meeting: Reiteration of the commitment to implement a full range of multilateral sanctions against North Korea; the full deployment of the Terminal High Altitude Area Defence (THAAD) system at an early date; and full and fair implementation of the South Korea-US free trade agreement.

First, the senators called on Trump to reaffirm the alliance and commitment to defence treaty obligations with South Korea. Trump had previously made conflicting remarks on Korean affairs.

He expressed his willingness to talk with the North's leader Kim Jong-un and then shifted to a hard-line stance, bolstering sanctions against the North and sending strategic assets to the Korean Peninsula. His aides even publicly said "all options are on the table."

Trump also did not hide his negative views toward South Korea. During his election campaign, he accused South Korea of not paying enough money for the hosting of American forces. He also criticised the Korea-US FTA as a deal that was only killing American jobs.

Since taking office, Trump has refrained from repeating those comments, but nevertheless, there is wariness among South Korean officials and the public.

In the letter, the senators noted that Trump's "maximum pressure" policy would only be effective with full cooperation from Seoul. They also urged Trump and Moon to seek a way to "expedite" the procedural review that is currently hindering the full deployment of the THAAD system.

In effect, the senators are pressing both Trump and Moon to reaffirm the alliance and agree on a joint stance on North Korea.

As things stand, it will not be easy for Trump and Moon to reach an agreement on all pending issues. But that does not give them room to expose differences on key issues such as the THAAD deployment and North Korea.

The situation on the Korean Peninsula is too grave to allow any crack in the alliance. Moon ought to dispel the worries and scepticism of Trump and other US leaders regarding his approach toward North Korea and relations with the US.

The Korea Herald is a member of The Straits Times media partner Asia News Network, an alliance of 22 news media entities.