The head of Homeland Security says recent attacks in New York and Minnesota underscore that homegrown attacks inspired by violent extremists are as much a threat to the United States as those directed by terrorists.

While all attacks are difficult to detect and prevent, Homeland Security Secretary Jeh Johnson says the U.S. and its allies continue to "take the fight militarily to terrorist organizations overseas."

Johnson tells Congress in prepared testimony that air strikes and special operations against the Islamic State terror group have killed many of its leaders. While it remains a threat, Johnson said the group has lost nearly half the populated areas it once controlled in Iraq and thousands of square miles in Syria.

Johnson and other officials are set to testify at a Senate hearing Tuesday.