By Colleen Jenkins

WINSTON-SALEM, North Carolina (Reuters) - The United States has agreed to drop charges of sexual assault brought against an Army general in exchange for him pleading guilty to lesser charges including "mistreatment" of his accuser, a junior officer, the general's lawyer said on Sunday.

Under the terms of an agreement with military prosecutors, the government will drop sex assault charges against Brigadier General Jeffrey Sinclair, as well as two other charges that would have forced him to register as a sex offender, lead defense attorney Richard Scheff said in an email.

Sinclair, 51, had been charged with forcible sodomy based on allegations by a female U.S. Army captain that could have sent the 27-year Army veteran to prison for life.

Sinclair is expected to enter his additional guilty pleas, including mistreatment of his accuser and failure to end an extramarital affair, in military court at Fort Bragg, North Carolina on Monday, his legal team said.

A spokeswoman for military prosecutors said they would withhold comment until the deal is announced in court.

It was unclear if the judge in the case would move directly to sentencing.

Sinclair already had pleaded guilty this month to military crimes of having an adulterous affair, asking junior female officers for nude photos and possessing pornography on his laptop while deployed in Afghanistan. Those offenses carry a maximum penalty of 15 years in prison and possible dismissal from the Army.

Sinclair's attorneys plan to argue that he should not face jail time on any of the charges, though he would accept retirement at a reduced rank, which would lower his pension.

Sinclair, the defendant in a rare court-martial against a top U.S. military officer, had vowed to fight charges that he sexually assaulted the 34-year-old female captain, saying their three-year relationship that spanned two war zones had been consensual.

His victim gave tearful testimony early this month accusing Sinclair of twice forcing her to engage in oral sex when she tried to break off the relationship and accusing him of threatening to kill her if she told anyone about the affair.

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The defense said it would prove Sinclair's accuser was not a credible witness. But his lawyers did not get a chance to cross-examine her after the trial was delayed indefinitely last week when the prosecution's case hit a stumbling block. A judge ruled that politics appeared to have fueled the Army's decision to reject an earlier plea offer made by Sinclair.

The judge also said he would allow Sinclair to renew an offer to plead guilty to lesser charges in exchange for the more serious being dismissed. That request had previously been rejected by military leaders at Fort Bragg.

A married father of two, Sinclair was stripped of command in southern Afghanistan in May 2012 as a result of the criminal allegations.

"It shouldn't have taken two years for them to come to this conclusion," Scheff said. "Like other persons cleared of false rape allegations, General Sinclair has borne enormous reputational and financial costs. These costs should factor into sentencing."

Sinclair's court-martial came at a time the U.S. military is grappling with how to handle rising sexual assault, after a report that incidents of unwanted sexual contact, from groping to rape, jumped by 37 percent in 2012, to 26,000 cases.

The U.S. Senate last week passed by an overwhelming margin a bill that would reform how the military handles sexual assault charges. The bill, subject to House approval, would strengthen prosecutors' role in advising commanders on whether to go to court-martial and eliminating the "good soldier" defense which allowed courts to reduced the sentence of offenders with strong military records.

(Writing by Victoria Cavaliere in New York; Editing by Scott Malone, Matthew Lewis and Marguerita Choy)