Officers from the North West Counter Terrorism Unit (NWCTU) detained a boy and a girl in the town of Mossley on Monday, Greater Manchester Police said in a statement.
Chief Superintendent Caroline Ball said there was no immediate threat to the community and that police had acted on information from law enforcement agencies.
"It is prudent we act on that information and carry out a thorough, professional investigation to determine the circumstances and details of the activity reported," she said.
No further details were immediately available.
