Five categories 

The president then outlined his plan on how to deal with some 240 prisoners held at Guantanamo, saying he will divide them into five categories.

First, "when feasible, we will try those who have violated American criminal laws in federal courts," he said.

Second, detainees who violate the laws of war will be tried at Military Commissions.

The president recently decided to restore the Bush-era Military Commission, which is a special military tribunal, to try some of the Guantanamo prisoners, while providing more legal protections for the defendants.

The third category, the president said, involves 21 detainees who have been released by the courts.

Fourth, there are detainees who can be transferred safely to another country.

So far, the administration's review team has approved fifty detainees for transfer.

Finally, there are detainees who "cannot be prosecuted yet who pose a clear danger to the American people" and may be held indefinitely after the closure of Guantanamo.

The president acknowledged that the fifth category "is the toughest issue we will face."

Nevertheless, he said the bottom line will be "I am not going to release individuals who endanger the American people."

Obama said the United States must have a thorough process of periodic review, so that any indefinite detention is "carefully evaluated and justified".

"Going forward, my administration will work with Congress to develop an appropriate legal regime so that our efforts are consistent with our values and our Constitution," he said, referring to a new system to hold some prisoners indefinitely.

US President Barack Obama speaks about America's national security while at the National Archives in Washington, May 21, 2009. [Xinhua]