BEIJING—A large part of North Korea’s underground nuclear test facility, which leader Kim Jong Un pledged to close, is unusable anyway due to the collapse of a cavity inside the mountain after the last blast there, Chinese scientists say.

Seismologists involved in a soon-to-be-published study also warned that another blast in the same spot and with similar yield could cause “environmental catastrophe.”

Another study led by Chinese seismologists and published this month also concluded that a secondary tremor shortly after the blast was caused by the cavity’s collapse, but didn’t judge whether the Punggye-ri test site could still be used.

Mr. Kim said last week he was suspending nuclear and missile tests and closing the Punggye-ri facility, where all six of his country’s nuclear tests took place. His announcement was welcomed by the U.S., South Korea and China as a positive step in the run-up to an inter-Korean summit on Friday and a planned meeting between Mr. Kim and U.S. President Donald Trump by June.

U.S. officials and North Korea watchers, however, are debating how meaningful Mr. Kim’s moves are. Some see them as major concessions and others, arguing that Punggye-ri is unusable, call them empty gestures designed to gain leverage with Washington and Seoul while remaining determined to retain his nuclear weapons.