The South Korean National Intelligence Service also told in during a closed-door meeting with lawmakers that there are signs that Pyongyang is also preparing for launching an intercontinental ballistic missile (ICBM), according to the Yonhap news agency.
The expectations for the North Korean nuclear and missile tests come amid South Korean concerns that Pyongyang may stage a provocation during the upcoming visit of US President Joe Biden to South Korea.
"It would not be abnormal for North Korea to launch a missile or conduct a nuclear test at one point, as signs (of such provocations) are detected and Pyongyang has almost completed its preparations," lawmaker Kim Byung-kee from the main opposition Democratic Party told Yonhap.
North Korea conducted its latest launch of an unknown projectile last week, marking the 16th test of missiles this year. The United States and its partners consider the tests to be provocative and in violation of international efforts to curb the Pyongyang's nuclear and ballistic missile programs.