“The alliance with the United States is and will always be the foundation of our diplomacy and national security,” Mr. Moon was quoted as telling Mr. Trump. “The alliance is more important than ever, given the rising uncertainty surrounding the Korean Peninsula.”
Mr. Moon’s comments appeared aimed at easing fears that his new liberal government and its eagerness for diplomatic and economic engagement with North Korea might create a rift with Washington.
Compared with his two conservative predecessors, who had emphasized a united front with Washington in punishing the North, Mr. Moon has often called for his country to take the lead in easing tensions on the divided peninsula through dialogue.
“I will do whatever it takes to help settle peace on the Korean Peninsula,” Mr. Moon said during a speech at the National Assembly, where he was formally sworn in on Wednesday. “If necessary, I will fly immediately to Washington.”
A day after winning the presidential election, Mr. Moon took office by reconfirming the broad changes he promised during his campaign, including curtailing the powers of the presidency and eliminating corrupt ties between government and business.
He also vowed to “get busy for the sake of peace on the Korean Peninsula.” Mr. Moon said he was also willing to travel to Pyongyang, the capital of North Korea, to meet with Mr. Kim. But he cautioned that for such a trip to take place, “the circumstances have to be right.” He had earlier said that dialogue would become difficult if the North raised tension with another nuclear test.
Mr. Moon is widely expected to introduce a modified version of Mr. Roh’s so-called sunshine policy of engaging North Korea with dialogue, humanitarian aid and joint economic projects.
The idea behind the sunshine policy was to build trust with the North so that it would negotiate away its nuclear and ballistic missile programs. But that policy was thrown out in the last nine years. The two last presidents in Seoul, both conservatives, joined hands with Washington to try to isolate Pyongyang with sanctions and pressure, as the North advanced its weapons programs by conducting a series of nuclear and missile tests.
Mr. Moon’s election signaled the return of the liberals to the center stage of South Korean diplomacy. They believe that sanctions alone have failed to persuade North Korea to give up its nuclear weapons programs. They also do not want South Korea to be dragged into a hegemonic struggle between the United States and China — an attitude exemplified by their ambiguous stance over the deployment in South Korea of an American antimissile battery, the Terminal High Altitude Area Defense system, known as Thaad.
Thaad went operational last week, despite angry protests from China, which said that Thaad’s powerful radar undermined its own security. Chinese consumers have begun a boycott of many South Korean brands. Many South Koreans fumed over the economic retaliation by China, considering it the price of protecting the alliance with Washington. But others accused the United States of foisting the weapon on their country, especially after Mr. Trump said that Seoul should pay $1 billion for the Thaad battery, contrary to an earlier agreement.
During his campaign, Mr. Moon said he would review the deployment, a stance he reaffirmed as president.
“I will engage in sincere negotiations with the United States and China to find a solution to the Thaad problem,” he said.
Mr. Moon’s victory on Tuesday capped months of political turmoil marked by the impeachment, ouster and arrest on corruption charges of his predecessor, Park Geun-hye. The country has been led by an acting president since Ms. Park was removed on March 10, and Mr. Moon took office without a customary two-month transition.
In his first day in office, Mr. Moon rushed to build his government, appointing Lee Nak-yon, a provincial governor, as his prime minister.
He also selected Suh Hoon, a former intelligence official, as director of the National Intelligence Service.
Mr. Suh has spent his career monitoring North Korea and was involved in the negotiations that resulted in the two summit meetings between the Koreas, the first in 2002 and the second in 2007. Mr. Moon said he expected Mr. Suh to play a role in resolving the North Korean nuclear crisis.
On Wednesday, Mr. Suh said that another inter-Korean summit meeting was “necessary,” but that it would be premature to discuss it when military tensions remained high on the peninsula.
“What we need the most is to find a breakthrough for resolving the North Korean nuclear issue,” Mr. Suh said. “When such conditions mature, I think we can go to Pyongyang.”
Mr. Moon’s is the first liberal government in South Korea in nearly a decade. Conservatives remained disgraced by Ms. Park’s downfall but deeply wary of Mr. Moon’s approach to the North, which they said could jeopardize the alliance with Washington.
Mr. Moon’s inaugural speech appeared to have been worded to ease such concerns while also putting a progressive stamp on foreign policy.
In the address, Mr. Moon emphasized “national unity” with his political opponents and vowed to make his government more transparent.
He said he would not take up residence in the Blue House, a symbol of what South Koreans call the “imperial presidency,” and instead work from a government building in the bustling center of Seoul to make his office more accessible to the people.
“I will become a clean president,” Mr. Moon said, referring to a succession of South Korean leaders, including Ms. Park, who have ended their presidency in disgrace because of corruption scandals. “I will become a president who can retire home as an ordinary citizen and is welcomed by neighbors.”