WASHINGTON  President Obama told human rights advocates at the White House on Wednesday that he was mulling the need for a “preventive detention” system that would establish a legal basis for the United States to incarcerate terrorism suspects who are deemed a threat to national security but cannot be tried, two participants in the private session said.

The discussion, in a 90-minute meeting in the Cabinet Room that included Attorney General Eric H. Holder Jr. and other top administration officials, came on the eve of a much-anticipated speech Mr. Obama is to give Thursday on a number of thorny national security matters, including his promise to close the detention center at the naval base in Guantánamo Bay, Cuba.

Human rights advocates are growing deeply uneasy with Mr. Obama’s stance on these issues, especially his recent move to block the release of photographs showing abuse of detainees, and his announcement that he is willing to try terrorism suspects in military commissions  a concept he criticized bitterly as a presidential candidate.

The two participants, outsiders who spoke on the condition of anonymity because the session was intended to be off the record, said they left the meeting dismayed.