U.S. National Security Advisor Jake Sullivan has expressed concern about the possibility of a seventh nuclear test by North Korea.

In an interview with CBS on Sunday, Sullivan said that while there do not appear to be any immediate indications that it is forthcoming, it would not be surprising if North Korea moves forward with another nuclear test.

He added that Washington is ready to engage on dialogue with Pyongyang without preconditions.

The security adviser went on to say that the U.S. watches the North’s intercontinental ballistic missile(ICBM) tests very closely and could coordinate extremely closely with its allies like South Korea and Japan to ensure that they are responding in lockstep to the threat.

Sullivan also called for China to join in the U.S.’ efforts to engage in dialogue with the North, saying that the regime’s continued testing is only creating circumstances in which the U.S. and its allies and partners have to step up their activities and posture to respond to the threat.

The remarks came four days after North Korea launched a Hwasong-18 solid-fuel ICBM.