The BBC's Andrew North says that the move means President Obama is "accepting political reality" US President Barack Obama is lifting the two-year freeze on new military trials for detainees at the Guantanamo Bay prison. Mr Obama also announced a new process for continuing to hold those detainees not charged or convicted but deemed too dangerous to free.
He said the measures would "broaden our ability to bring terrorists to justice".
Mr Obama had pledged in January 2009 to close the prison within a year.
"The American system of justice is a key part of our arsenal in the war against al-Qaeda and its affiliates," Mr Obama said in a statement.
He added that military commissions "ensure that our security and our values are strengthened".
US defence secretary Robert Gates issued an order on Monday revoking his previous suspension on the "swearing and referring of new charges in the military commissions".
New military commissions at the prison, which holds a number of top suspects from the 9/11 attacks and other strikes against the US, have been suspended since January 2009.
The White House said Mr Obama remained committed to the eventual closure of Guantanamo Bay.
'Humane treatment' Mr Obama ruled in an executive order that detainees would have the right to a periodic review of the reasons for their detainment. The order was designed to ensure inmates detained indefinitely without trial were only kept in prison when it was lawful and necessary to do so, the White House said.
"I am announcing several steps that broaden our ability to bring terrorists to justice, provide oversight for our actions and ensure the humane treatment of detainees," Mr Obama said.
US Attorney General Eric Holder said the executive order "strengthens the legal framework under which we will continue to detain those individuals who are at war with our country and who pose a significant threat" to US security.
The president also reserved the right to try some suspects from Guantanamo Bay prison in federal prisons on the US mainland, a move that has been repeatedly opposed by members of Congress.
Mr Obama signed a sweeping defence bill in January, which blocks the use of defence department funding to transfer Guantanamo suspects to the US for trial.
The White House said on Monday it would now work to overturn the law.
'Combating terror' The Obama administration said it would allow certain trials to resume, having carried out key reforms such as a ban on the use of statements taken under "cruel, inhuman or degrading treatment" and the introduction of a better system for handling classified information.
"With these and other reforms, military commissions, along with prosecutions of suspected terrorists in civilian courts, are an available and important tool in combating international terrorists," the Obama administration said.
The first trial likely to proceed under the new order would involve Abd al-Rahim al-Nashiri, a Saudi accused of planning the 2000 bombing of the destroyer USS Cole in Yemen.
He has been imprisoned at Guantanamo since 2006.
Some 172 detainees remain at Guantanamo, of whom 35 have been recommended for prosecution.
Secretary of State Hillary Clinton also said on Monday that the US "remains committed to the development and maintenance of humanitarian protections" in armed conflicts.
Mr Obama's announcement on Monday is being seen as his latest attempt to work around "war on terror" policies put in place by the previous Bush administration.