U.S. President Barack Obama says there have been gains against so-called Islamic State in Iraq and Syria but warned the group still poses a threat, BBC News reports.

As the jihadist group lost ground, he said, there were signs it was shifting to attacks abroad.

"The possibility of a lone actor or a small cell that kills people is real," he said, adding that networks in the U.S. could be activated.

Obama was giving an update to reporters after a Pentagon meeting.

Speaking after the meeting with top military and national security officials on Thursday, Obama said IS was "not invincible".

"In fact, they are inevitably going to be defeated," the president said, predicting that the jihadist group would lose its self-proclaimed capital of Raqqa in Syria, in addition to their stronghold of Mosul in Iraq.