A senior US official says the leaders of the United States, Japan and South Korea are expected to agree to set up a three-way hotline when they meet at a trilateral summit on Friday.

Kurt Campbell, the National Security Council Coordinator for the Indo-Pacific, made the remarks at a think tank event in Washington on Wednesday. The summit will take place near the capital.

He said the US relationship with Japan and South Korea would be a "defining trilateral relationship for the 21st century."

Campbell said the leaders will announce "a very ambitious set of initiatives that seek to lock in trilateral engagement, both now and into the future."

He said the three are also expected to reach an agreement to make the trilateral summit an annual event, and to deepen cooperation in a wide range of fields, including security, technology and education.