Staff Sgt. Robert Bales plans to enter guilty pleas before a military judge next week, criminal defense attorney John Henry Browne told The New York Times.
Military prosecutors at Joint Base Lewis-McChord near Tacoma, Wash., have agreed to accept the plea agreement, Browne said.
Base spokesman Lt. Col. Gary Dangerfield said he could not confirm a deal had been reached or even if the two sides were negotiating a plea bargain.
"John Henry Browne is probably not saying it just to be saying it," Dangerfield told the Times. "But I can't confirm whether they are in talks."
A plea hearing is tentatively scheduled for Wednesday, he said.
Bales, 39, held at the base's Northwest Joint Regional Correctional Facility, is charged with 16 counts of premeditated murder and six counts of assault and attempted murder in the March 11, 2012, killings.
He is charged with walking off a small outpost in Kandahar province's Panjwai district before dawn that day and shooting or stabbing to death 16 people -- including nine children, four men and three women -- in two villages. Six other people were wounded.
Eleven of the dead were from the same family. Witnesses said the family members were shot in the head, stabbed and then gathered into a room and set on fire. Burning the bodies is considered desecration under Islamic law.
Bales was apprehended as he returned to base.
U.S. and allied forces apologized for the massacre -- widely considered one of the most gruesome U.S. soldier atrocities in the Afghanistan war -- and promised a thorough and quick investigation.
U.S. President Barack Obama called the incident "absolutely tragic and heartbreaking."
At Bales' November Article 32 hearing, similar to a preliminary hearing in civilian law, a fellow soldier testified Bales told him he had "shot up some people."
A lab examiner testified Bales had blood from at least four people on his clothes when taken into custody.
U.S. Army prosecutors said in December they would seek the death penalty for Bales.
Six U.S. military members are on death row, but none has been executed since Pfc. John A. Bennett was hanged in 1961 after being convicted for the rape and attempted murder of an 11-year-old Austrian girl.