The information, which could help track down current and former Islamic State group jihadists, was included in forms which new recruits had to fill out, British and German media reported.
The documents contain details like names, dates of birth and phone numbers for people from 51 countries including from Britain, northern Europe, the Middle East, north Africa, the United States and Canada.
British broadcaster Sky News said it had shared with British security services documents with the identities of some 22,000 IS group members.
"If it can, then we would welcome that," she said, adding that the government had not been aware of the story before it was reported by Sky.
More From This Section
German federal police spokesman Markus Koths said there was "a very high probability" that documents reported by German media as identifying German jihadists were genuine.
"We are therefore taking them into account as part of our law enforcement and security measures," he added.
"Our security services have received such information," he said.