US President George Bush has refused to rule out military tribunals for inmates at Guantanamo Bay detention centre.
Many Guantanamo detainees have been held for years
His administration was dealt a blow on Thursday when the Supreme Court ruled it had overstepped its authority in setting up the tribunals.
But Republican senators immediately began planning how to win congressional approval for new tribunals.
The ruling came in response to a case brought by Osama Bin Laden's ex-driver, Salim Ahmed Hamdan.
He is one of 10 Guantanamo inmates facing a military tribunal, but demanding to be tried by a civilian tribunal or court martial, where proceedings would be more open and defendants would have greater access to the evidence against them.
His lawyer said he was "awe-struck" at the court's ruling.
The Cuba-based facility currently holds about 460 inmates, mostly without charge, whom the US suspects of links to al-Qaeda or the Taleban.
No 'sweeping mandate'
In its ruling, the court said military tribunals contravened both the Geneva Convention on the treatment of prisoners, and the US code of military justice.
It also ruled that the tribunals were not expressly authorised by any congressional act, and there was no "sweeping mandate for the president to invoke military commissions whenever he deems them necessary".
But the ruling does not demand the release of prisoners held at Guantanamo, and it does hold out the possibility of coming up with another way of trying those held.
President Bush told reporters he promised to take the findings of the court "very seriously". But he signalled he might seek congressional approval to resurrect the tribunals.
"To the extent that there is latitude to work with the Congress to determine whether or not the military tribunals will be an avenue in which to give people their day in court, we will do so," he said.
"The American people need to know that this ruling, as I understand it, won't cause killers to be put out on the street."
Within minutes of the court ruling, a small group of Republican senators were working the phones trying to sort out the mess, reports the BBC's Justin Webb in Washington.
A former military lawyer who is leading the efforts to salvage the tribunal system, Senator Lindsey Graham, predicted that the Senate would begin work on ideas for new tribunals within weeks and vote on the plan in September.
Meanwhile, Sen Arlen Specter, a moderate Republican who chairs the powerful judiciary committee, introduced an "Unprivileged Combatant Act" which would, he said, balance "the need for national security with the need to afford detainees with sufficient due process".
White House spokesman Tony Snow underlined the administration's resistance to abandoning the special courts.
"Nobody gets a 'get out of jail free' card," he said.
Mr Hamdan had success in his first legal outing, in the US District Court in Washington, which ruled that he could not face a military trial unless he had previously been found not to be a prisoner of war under the Geneva Convention.
He claims POW status, but like all camp prisoners, he is denied this and is instead designated an "unlawful combatant" by the Bush administration.
Hamdan was a driver for Osama Bin Laden
However, an appeal court reversed this decision and said Mr Bush had the authority to order the trials.
This latest decision was welcomed by human rights groups, lawyers for inmates and some politicians including senior Democrat Senator Carl Levin.
"The Supreme Court has once again demonstrated its vital constitutional role as a check and balance on the actions of the executive and legislative branches of government," he said in a statement.