Hours after taking office on Tuesday, United States President Barack Obama ordered military prosecutors in the Guantanamo war crimes tribunals to ask for a 120-day halt in all pending cases.
Military judges were expected to rule on the request on Wednesday at the US naval base in Guantanamo Bay, Cuba, an official involved in the trials said.
The request would halt proceedings in 21 pending cases, including the death penalty case against five Guantanamo prisoners accused of plotting the September 11 hijacked plane attacks that killed nearly 3,000 people.
Prosecutors said in their written request that the halt was "in the interests of justice."
Mr Obama became the 44th president of the United States on Tuesday.
He has pledged to shut down the Guantanamo prison camp that was widely seen as a stain on the United States' human rights record and a symbol of detainee abuse and detention without charge under the administration of his predecessor, former President George W Bush.
Human rights activists and military defence lawyers had urged Mr Obama to halt the special tribunals that are formally known as military commissions and urged him to move the prosecutions into the regular US courts for trial under long-established rules.
The request said freezing the trials until 20 May would give the new administration time to evaluate the cases and decide what forum best suits any future prosecution.
About 245 foreign captives are still held at the detention centre that opened in January 2002.
The Bush administration had said it planned to try 80 prisoners on war crimes charges, but only three cases have been completed.