WASHINGTON (AP) _ An Army warrant officer admitted at the opening of his court-martial today that he was involved in a five-year conspiracy to provide classified military information to Warsaw Pact nations.

Warrant Officer James W. Hall III, 30, of New York City, pleaded guilty to all 10 counts of espionage, attempted espionage and failing to obey Army regulations that were lodged against him after his arrest last December in Georgia.

Col. Howard C. Eggers, the military judge presiding at the court martial at Fort McNair here, did not immediately accept Hall's guilty plea. Under Army regulations, he will hear the prosecution's case against Hall and defense witnesses before deciding Hall's guilt and punishment.

Court officials said Eggers' final verdict and sentence probably will not come before Friday.

Under a defense-initiated pre-trial agreement disclosed during the court martial today, Hall pleaded guilty to all charges against him in exchange for the government agreeing not to seek the death penalty.

The rest of the government's agreement on a sentence for Hall was not disclosed, including the maximum sentence Hall will receive. Under Army procedures, Eggers will not see the sentencing agreement until after he pronounces sentence on Hall, and if his sentence is more severe than the agreement, the agreement will prevail.

Vaughan E. Taylor, Hall's civilian attorney, said Hall would not ''comment at any time now or in the future.''

''He has authorized me to inform you that he is truly sorry for whatever dam / uninvolved and unaware of his activities and Hall will spend the remainder of his life working to prove that he eventually deserves to be reunited with his wife and children,'' the statement said.

Taylor said Hall intends to tell Eggers, during the closed portion of his court-martial, ''all the actions he has taken to neutralize this situatiuon and then throw himself on the mercy of the military judge.''

Taylor said through a military spokesman that the defense would have no comment on Hall's motivation for passing classified information to representatives of East Germany and the Soviet Union until after he is questioned by Eggers.

Hall was arrested Dec. 21 near Savannah, Ga., after admitting to an FBI undercover agent that he has been passing intelligence information to the Warsaw Pact countries.

Hall, an intelligence analyst with a top-secret security clearance, was accused of delivering exposed film and documents dealing with communications intelligence and war plans.

At the time of his arrest, Hall was assigned to the 24th Infantry Division and worked as an intelligence analyst in the Army's G-2 intelligence section. He joined the Army in 1976 and spent most of his career in Europe working as an analyst of intercepted radio and telephone traffic and coded messages.