A top US army prosecutor in charge of sexual assault cases has been accused of allegedly groping and assaulting a female lawyer.

A defence official has confirmed that Lt. Col. Joseph "Jay" Morse has been suspended from his position as the chief of the army's Trial Counsel Assistance Program at Fort Belvoir, which supervises attorneys who prosecute sexual assault cases.

The reported attack took place in 2011, before Morse took his post. As initially reported by the military newspaper Stars and Stripes, he allegedly attempted to kiss and grope the female against her will while both were attending a sexual assault legal conference in Alexandria, Virginia. No charges have yet been filed on the case.

Speaking to ABC news, Col. Dave Patternson said: "We can confirm that this matter is currently under investigation and the individual in question has been suspended from duties pending the outcome of the investigation."

He added that no additional information could be provided as the investigation is still open. Morse has not yet commented on the case.

Morse was also the lead prosecutor in the case against Staff Sgt Robert Bales, who pleaded guilty to killing 16 Afghan civilians.

The news came on the same day the Senate rejected a bipartisan bill which would have removed the military chain of command from referring sexual assault cases for prosecution. The Military Justice Improvement Bill, written by Senator Kirsten Gillibrand, junior United States Senator from New York, fell five votes short of the 60 needed.

This is the third instance in the past year in which a military official involved in the prevention or handling of assault cases has been accused of assault. On Thursday, Brig. Gen Jeffrey Sinclair pleaded guilty to seeking nude photographs of junior officers and using pornography in Afghanistan. Sinclair admitted having an adulterous affair, but is challenging charges accusing him of sexually assaulting a female captain during a three-year relationship.

In 2013, Lt. Col. Jeffrey Krusinski, who was in charge of the Air Force's Sexual Assault Prevention and Response Office, was acquitted of charges of alleged assault and battery against a woman in the car park of a bar near the Pentagon.

Preliminary figures released by the Pentagon this week have revealed there were around 5,400 reports of sexual assault last year. The number had risen from 3,774 in 2012, marking a 60% increase in reported alleged incidents. Around 11% of the reported sexual assaults occurred before the victim entered military service.

Last year, a survey released by the Pentagon estimated there may have been 26,000 instances of "unwanted sexual contact" in 2012. The research was conducted from a sample of military service members, who were asked if they had experienced assault from touching to rape.