U.S. National Security Adviser John Bolton referred to North Korea as a "pressing" challenge after holding talks with Singapore's top diplomat.

Bolton tweeted on Thursday that he had a great meeting with Singaporean Foreign Minister Vivian Balakrishnan in Washington to strengthen robust cooperation and the strategic partnership of their nations. 

He added that Singapore is the U.S.' valued partner on "pressing challenges," including North Korea and Iran.

Bolton rarely mentions the North in his tweets and his recent post appears to indicate he discussed North Korean issues during the meeting.