South Korea says its president will hold summit talks next week with Chinese President Xi Jinping to try to improve ties strained over the deployment of a U.S. missile-defense system.

Senior presidential official Lee Sang-chul told a televised briefing Tuesday that President Moon Jae-in and Xi will talk on the sidelines of an annual regional forum in Vietnam next week.

Lee says the agreement is part of efforts by the two countries to put bilateral ties on a normal track.

In response to the missile-defense system's deployment, China has issued angry rhetoric and South Korean businesses operating in China have suffered economic retaliation.

China believes the system poses a threat to its own security. According to AP Seoul and Washington say it is purely aimed at defending South Korea against North Korean threats.

Last week North Korea ordered mass evacuation drills as the Democratic People's Republic of Korea (DPRK) prepares for an attack.

US Secretary of Defense James Mattis who was visiting the South Korean side of the inter-Korean border demilitarised zone (DMZ), one of the world's most heavily-militarised frontiers, between North and South Korea, told reporters: "North Korean provocations continue to threaten regional and world peace and, despite unanimous condemnation by the United Nations' Security Council, they still proceed.

"Ultimately, our diplomats have to be backed up by strong soldiers and sailors, airmen and Marines. So they speak from a position of strength, of combined strength, of alliance strength, shoulder-to-shoulder," he added.