A senior foreign policy adviser to Chinese President Xi Jinping stressed the importance of relations with South Korea on Friday as the two countries attempt to mend a rift in ties over the deployment of a U.S. missile-defense system to guard against North Korean threats.

State Councilor Yang Jiechi made the remarks at the opening of a second day of meetings between Chinese officials and special envoy Lee Hae-chan, a former South Korean prime minister.

They were expected to discuss ways of containing North Korea’s missile and nuclear weapons activities as well as the economic fallout over the deployment in South Korea of the U.S. missile defense system called THAAD.

Mr. Lee said South Korean President Moon Jae-In had dispatched him to China to keep communications open between the two countries “at a critical time.”

Seoul and Washington have argued that the missile system is aimed at North Korean aggression, while China sees it as a threat to its own security. It has retaliated against Seoul by suspending visits to South Korea by Chinese tour groups and trips to China by South Korean entertainers. South Korean businesses have faced boycotts, especially the retail group Lotte which provided the land on which the system is being constructed.

China’s top diplomat, Foreign Minister Wang Yi, on Thursday reiterated calls for its dismantling.

“We’re now at a crossroads in our relations,” Wang told Lee as he urged the new South Korean administration to make a decision to “remove the obstacles” that stand in the way of healthy ties between the two Asian economic powerhouses.

In recent weeks Beijing and Seoul have signaled a desire to repair relations following the election of Mr. Moon, who has taken a friendlier stance toward China than his conservative predecessor. Although he has sometimes criticized the THAAD deployment, Mr. Moon has not said he will remove it.

When Chinese President Xi Jinping called Mr. Moon last week to congratulate him on his election, Mr. Moon reportedly asked Mr. Xi for help in ending the economic retaliation that has taken a toll on South Korean businesses.

Beijing has maintained its hard line, and in an editorial on Thursday, the Communist Party newspaper Global Times said China’s opposition “cannot be traded for the new government’s friendly posture toward China.”

“Stopping the deployment of THAAD is the bottom line of China,” the newspaper said. “Seoul needs to make a choice between deploying THAAD and resuming Sino-South Korean relations. It should not hope to have it both ways.”