ASHINGTON, Jan. 9 — The Supreme Court, significantly expanding its review of the Bush administration's treatment of those deemed "enemy combatants," agreed Friday to hear a challenge by an American of Saudi descent, Yaser Esam Hamdi, to his open-ended confinement at a military brig in South Carolina.

A federal appeals court ruled last year that regardless of citizenship, a person "captured in a zone of active combat" in a foreign country could not gain access to court to contest the factual basis for his detention. Mr. Hamdi was fighting with the Taliban in Afghanistan in late 2001, the government says, when his unit surrendered to the Northern Alliance, with which American forces were aligned. He has been held for two years without charges and, until last month, without access to a lawyer.

In the decision last year, the United States Court of Appeals for the Fourth Circuit, in Richmond, Va., accepted a two-page declaration by a Pentagon official as a "sufficient basis" for concluding that Mr. Hamdi's confinement was within the president's constitutional authority as commander in chief.

"No further factual inquiry is necessary or proper," the appeals court said.

The appeal consequently raises important questions not only about the rights of detainees but also about the authority of the federal courts to monitor the executive branch's determinations to classify an American citizen as an enemy combatant. An appeal that the Supreme Court had already agreed to hear, on behalf of 16 foreigners detained as enemy combatants at the United States naval base at Guantánamo Bay, Cuba, raises similar issues of judicial authority and access to courts.

Both cases will be argued in April, possibly along with an appeal that the Bush administration informed the court this week it would soon file in a case involving Jose Padilla, another United States citizen deemed an enemy combatant. With rulings expected by early summer, there is a sense that after a lengthy hiatus during which the Supreme Court remained on the sidelines, events are moving quickly toward answering the legal questions raised by the administration's assertive responses to the terrorist attacks of Sept. 11, 2001.

The Hamdi case in particular "has been the rallying cry for all those people concerned about how far the president can go," Michael Greenberger, a law professor at the University of Maryland and director of its Center for Health and Homeland Security, said in an interview Friday.

"The court," Professor Greenberger added, "has taken the case that goes to the very heart of the nationwide debate about whether the administration has gone too far."

Frank W. Dunham Jr., the federal public defender who filed the appeal on Mr. Hamdi's behalf, said in his brief that "by permitting the military to act as the judge of its own cause," the appeals court had "emboldened the executive branch, weakened the judiciary and threatened the elemental constitutional protections of the citizen."

The Bush administration strongly opposed the appeal, Hamdi v. Rumsfeld, No. 03-6696, telling the justices that the appeals court had properly understood that "fundamental separation-of-powers principles," as well as "the time-honored military practice of detaining captured combatants in wartime," dictated a very limited judicial role in reviewing the determination of Mr. Hamdi's status. Mr. Hamdi was "an archetypal enemy combatant" in any event, the government's brief said.

After the court's action Friday, the administration issued a statement declaring that "the Justice Department will vigorously defend the president's authority to capture and detain enemy combatants."

The Supreme Court's decision to hear the appeal over the administration's opposition mirrored its action two months ago when, in the Guantánamo case, it granted the detained foreigners' appeal despite — or perhaps because of — the administration's assertion that those detainees' status was a question "constitutionally committed to the executive branch" with which the federal courts should not interfere.

It is clearly too soon to say whether by accepting these cases the court was sending a signal about their ultimate resolution. Though the administration's critics were quick to read favorable tea leaves in the latest development, it is equally plausible to assume that justices across the ideological spectrum simply concluded that the cases raised issues of historic dimension meriting the court's consideration. Chief Justice William H. Rehnquist, who wrote a book about civil liberties in wartime, "All the Laws but One," published in 1998, is clearly fascinated by the subject.

At the same time, it is evident that the administration went to unusual lengths to dissuade the court from accepting the Hamdi case. On Dec. 2, the day before the administration's reply to Mr. Hamdi's initial brief was due at the court, the Pentagon announced that "as a matter of discretion and military policy," it would now give him access to a lawyer, a development that Solicitor General Theodore B. Olson then noted in the administration's brief.

Mr. Dunham, the public defender, replied in a subsequent brief that this concession meant little because the appeals court's conclusion that Mr. Hamdi "could not challenge the facts upon which his detention is based effectively eliminated any litigation-related basis for communication" with a lawyer — in other words, under the appeals court's view of the case, there would be little if anything for a lawyer to do.

Then this week, with the justices scheduled to take up the Hamdi appeal for the first time at their weekly conference on Friday, Solicitor General Olson filed a supplemental brief asking the court to defer its consideration in order to give him time to file an appeal for the government in the Jose Padilla case.

In that case, the United States Court of Appeals for the Second Circuit, in New York, ruled last month that the government lacked authority to hold Mr. Padilla in military custody. He was arrested in May 2002 after arriving at O'Hare International Airport in Chicago on a flight from Pakistan and was initially held as a material witness on suspicion of involvement in a plot to detonate a "dirty bomb" in the United States.

The Second Circuit did not challenge the Fourth Circuit's decision in the Hamdi case, concluding instead that having been seized on American soil rather than in a combat zone, Mr. Padilla was in a different situation and could not be detained as an enemy combatant. In his supplemental brief this week, Mr. Olson told the justices that because the Second Circuit decision "incorrectly resolves issues of extraordinary public significance," the government would file a Supreme Court appeal by Jan. 20, well short of the 90 days permitted by the court's rules and in time for an April argument.

While the administration urged the court to wait for the Padilla appeal and to accept that case instead of the Hamdi case, it is quite likely that the justices will hear and decide both.