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 congratulatory phone call U.S. President Donald Trump agreed with Moon to cooperate 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.