South Korea’s new liberal President Moon Jae-in was sworn in on Wednesday and vowed to immediately tackle 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 he would begin efforts to defuse security tensions on the Korean peninsula and negotiate with Washington and Beijing to ease a row over a U.S. missile defense system being deployed in the South.

In a phone call congratulating Moon’s election, U.S. President Donald Trump agreed with the new South Korean leader to cooperate on North Korea’s nuclear issue and invited him to visit Washington, the South Korean presidential office said.

Trump reaffirmed the U.S.-South Korea alliance was strong and said North Korea’s nuclear issue was a difficult problem but one that could be resolved, the Blue House said in a statement.

In his first key appointments, Moon named two liberal veterans with ties to the “Sunshine Policy” of engagement with North Korea from the 2000s to the posts of prime minister and spy chief.

Moon, 64, also pledged to sever what he described as the collusive ties between business and government that have plagued many of South Korea’s family-run conglomerates, known as chaebol, and vowed to be incorruptible.