South Korea's new liberal President Moon Jae-in has vowed to tackle immediately the difficult tasks of addressing North Korea's advancing nuclear ambitions and soothing tensions with the United States and China.
Moon said in his first speech as president on Wednesday he would begin efforts to defuse security tensions on the Korean peninsula and negotiate with Washington and Beijing to ease a row over a US missile defence system being deployed in the South.
In a congratulatory phone call US President Donald Trump agreed with Moon to co-operate on the North Korean nuclear issue and invited him to visit Washington, the South Korean presidential office said.
The White House said the two agreed to continue to strengthen the alliance between their countries.
The South Korean statement said Trump said North Korea's nuclear problem was difficult, but one that could be resolved.
North Korea is believed to be preparing for a sixth nuclear test and is working to develop a nuclear-tipped missile capable of reaching the United States, presenting Trump with perhaps his most pressing security issue.
US officials have expressed concern that Moon's advocacy of engagement with North Korea could undercut US efforts to increase pressure through further isolation and sanctions.
The Trump administration has also been dismayed by Moon's questioning of deployment of a US missile defence system in South Korea and expects some friction in ties, although US officials say the new president may moderate his stances in office and the effect on the alliance will be limited.
In his first key appointments, Moon named two liberal veterans with ties to the 2000s "Sunshine Policy" of engagement with North Korea to the posts of prime minister and spy chief.
Suh Hoon, a career spy agency official who was instrumental in setting up two previous summits between the North and South, was named to head the National Intelligence Service.
Lee Nak-yon, a regional governor who was a political ally of two former presidents who held summits with North Korea in 2000 and 2007 was nominated as prime minister. His appointment requires parliamentary approval.
Moon was expected to fill the remaining cabinet and presidential staff appointments swiftly to bring an end to a power vacuum left by the removal of Park Geun-hye in March in a corruption scandal that rocked South Korea's business and political elite.
"I will urgently try to solve the security crisis," Moon said in the domed rotunda hall of the parliament building. "If needed, I will fly straight to Washington. I will go to Beijing and Tokyo and, if the conditions are right, to Pyongyang also."
Suh said Moon could go to Pyongyang if it was clear the visit would help resolve the North Korean nuclear crisis and ease tensions.
North Korea is likely to welcome Moon's election but its state media made no mention of his victory on Wednesday.
