WASHINGTON — A new batch of classified documents that appear to detail American national security secrets from Ukraine to the Middle East to China surfaced on social media sites on Friday, alarming the Pentagon and adding turmoil to a situation that seemed to have caught the Biden administration off guard.

The scale of the leak — analysts say more than 100 documents may have been obtained — along with the sensitivity of the documents themselves, could be hugely damaging, U.S. officials said. A senior intelligence official called the leak “a nightmare for the Five Eyes,” in a reference to the United States, Britain, Australia, New Zealand and Canada, the so-called Five Eyes nations that broadly share intelligence.