"We are meeting here... amid reports that al Qaeda is again seeking to harm Americans. This should surprise no one.
But it is a reminder of the continuing stakes in our struggle against violent extremism.
"We are taking this threat seriously, and federal, state and local authorities are taking all necessary steps to address it," Clinton said in a speech at the John Jay College of Criminal Justice in Manhattan here.
She said the death of Osama bin Laden has put al Qaeda on the "path to defeat" but "we must be clear about the threat that remains.
"Cities such as London and Lahore, Madrid and Mumbai have been attacked since 9/11," Clinton said.
Thousands of innocent people most of them Muslims have been killed in these attacks and even the best of efforts have not guaranteed "perfect security," she said.
While al Qaeda's core leadership in Afghanistan and Pakistan has been weakened significantly, the terror group can still conduct regional and international attacks and inspire others to do so, Clinton noted.
The threat has become more geographically diverse, with much of al Qaeda's activity devolving to its affiliates around the world. .