French and American targets far outside the BND's officially-stated mission were among those singled out for surveillance, Spiegel Online reported.
Several thousand such 'selectors' – search terms such as a telephone number, email address or IP address – were among the BND's targets, sources said.
'Selectors' are used to identify information relevant to specific targets among the vast amounts of data collected by intelligence agencies like the BND, Britain's GCHQ and the American NSA.
MPs in the Parliamentary Oversight Committee – which is supposed to check up on the work of Germany's secret services – learned of the surveillance from the federal government on Wednesday evening.
Now a task force of parliamentarians is set to visit BND headquarters in Pullach, Bavaria to check on the selector list themselves and question the agency's employees.
Their main question will be who knew about the illegal surveillance and who ordered it.
The BND has faced serious public scrutiny in 2015, after it emerged earlier in the year that it had been conducting surveillance on European allies, including France, and German companies such as Siemens on the orders of American colleagues at the NSA.
It's the latest in a series of controversies over co-operation across borders between intelligence agencies that saw Britain attempt to intimidate German MPs out of investigating the BND in February.
The MP heading a special parliamentary inquiry into the BND's spying also suspected that his phone had been hacked in March.
When he sent it to the government IT service to be checked, the package was physically interfered with before it arrived.