WASHINGTON-- As U.S. President Donald Trump returns home from a long Asia trip on Tuesday, experts here said his visit helped narrow gaps between Seoul and Washington over North Korea but still left key challenges on trade and security issues ahead.

In last week's summit, Trump and South Korean President Moon Jae-in agreed to beef up deterrence against North Korea's nuclear threats.

In an address to the National Assembly, Trump warned the communist regime against carrying out additional nuclear and missile tests but also offered a path to a better future.

"In combination with President Moon Jae-in's efforts, I think President Trump's visit to Korea contributed to a higher level of coordination between the United States and the ROK on dealing with North Korea," Alan Romberg, director of the Stimson Center's East Asia Program, said. ROK is the acronym for South Korea's official name, the Republic of Korea.

"Seoul's commitment to strengthen its efforts to press the North and Washington's greater clarity that it will not initiate use of force but will only respond to DPRK aggression were important factors in bringing the two sides, and two leaders, even closer together," he said, referring to North Korea by the acronym for its official name, the Democratic People's Republic of Korea.

Moon and Trump had early on shown signs of a rift in their approach to Pyongyang. While the South Korean former human rights lawyer has long advocated engagement with the regime, the American former businessman has mostly employed tough rhetoric and personal insults directed at North Korean dictator Kim Jong-un.

Once Trump even described Moon's policy as "appeasement."

"Trump moderated his extreme positions on North Korea during the course of his trip, suggesting that in time there may be a way to negotiation (with Pyongyang)," said Douglas Paal, vice president for studies for the Carnegie Endowment for International Peace Asia Program.

"I think he contributed to a better bilateral relationship with Seoul, but I am not sure there was progress in resolving issues over KORUS or China's relations with South Korea over THAAD."

KORUS is the South Korea-U.S. free trade agreement that Trump has slammed as "horrible" over what he sees as its role in widening the U.S. trade deficit with Seoul.

Negotiations are currently underway to amend the five-year-old deal.

"I think there is at least a 50-50 chance Trump will kill KORUS," said Robert Manning, senior fellow at the Atlantic Council. Trump "has a point" in that there are problems with South Korea's implementation of the deal, but it is "easily fixable," he added.

THAAD, a U.S. missile defense system deployed in South Korea this year, has been a source of friction in relations between Seoul and Beijing. China views the system as a threat to its security despite the allies' assurances that it is purely defensive.

But days before the Moon-Trump summit, South Korea and China announced an agreement to normalize their ties. That included Seoul's pledge to not host additional batteries of THAAD.

It raised some eyebrows, according to Evans Revere, a former principal deputy assistant secretary of state for East Asian and Pacific affairs.

"It seemed to foreclose the ROK's future security options and preemptively close the door on future THAAD deployments in the ROK even if these deployments are deemed essential because of the need to protect U.S. forces and the ROK from a rising threat from North Korea," he said.

The former diplomat also relayed what he said was a concern among many U.S. experts that South Korea rejected Trump's "Indo-Pacific" initiative, which seeks to downplay China's dominance in the "Asia-Pacific" region.

"President Moon's media interview describing the ROK's goal as 'balanced' diplomacy between the U.S. and China caused discomfort among many American Korea hands," Revere said.

"Each of these developments is a reminder that, below the surface of ongoing U.S.-ROK cooperation and coordination, there are lingering differences and disconnects that will need to be acknowledged and managed carefully in order to keep bilateral ties on a good track."

Source: Yonhap News Agency