NEW HAVEN, Conn. — The U.S. military has failed to upgrade the discharges of Vietnam veterans who developed post-traumatic stress disorder, resulting in stigma and loss of benefits, according to a federal lawsuit filed Monday.

Five Vietnam veterans and three veterans organizations are suing the Army, the Navy and the Air Force in Connecticut. The veterans say they suffered PTSD before it was recognized and were discharged under other-than-honorable conditions that made them ineligible for benefits.

The lawsuit, which seeks class-action status to represent tens of thousands of veterans, says the military has systematically denied applications for upgrades involving evidence of PTSD.

“Unfortunately, the Pentagon has refused to correct the decades of injustice experienced by tens of thousands of veterans who suffer from PTSD but were discharged before it was a diagnosable condition,” V Prentice, a law student intern in the Veterans Legal Services Clinic at Yale Law School, which represents the plaintiffs, said in a news release. “This action seeks to compel appropriate action by the military and to finally secure justice for these veterans.”

Lt. Col. Damien Pickart, a Pentagon spokesman, said the department doesn’t comment on pending litigation. The Department of Defense has said the agency is committed to addressing concerns related to PTSD and has taken numerous steps, including conducting PTSD assessments of service members at military treatment facilities.

A spokesman for the U.S. Attorney’s office also declined comment.

Conley Monk, a New Haven resident who served in the Marine Corps in Vietnam, developed PTSD after suffering traumatic events including a barrage of enemy mortar rounds and the gassing of his unit, according to the lawsuit. He later experienced flashbacks and hyper vigilance when he was stationed in Japan and went absent without leave, the suit said.

“When I was in high school, I worked at the VA hospital in the kitchen as a dishwasher. But after I came home from Vietnam, I couldn’t even get my job back as a dishwasher because of my bad paper,” said Monk, one of the plaintiffs. “My discharge status has been a lifetime scar. If I were discharged today, my PTSD would be recognized and treated and I wouldn’t be punished for having a service-connected medical condition.”

A proposed class-action lawsuit over the issue was filed in 2012 as part of a claim involving a Vietnam veteran, but that veteran’s case was settled. The class-action part of the case was not decided, the law students said.

Since 1993, only 4.5 percent of about 375 applications for discharge upgrades involving PTSD have been granted for Vietnam veterans, according to the lawsuit, which seeks what it calls consistent and medically appropriate standards for considering the effects of PTSD when determining whether to upgrade a discharge.

“Tens of thousands of brave and honorable Vietnam veterans with post-traumatic stress have been doubly injured by the black mark of an other than honorable discharge, resulting in unjustly denied support, services and benefits,” said U.S. Sen. Richard Blumenthal, D-Conn. “These heroic veterans are long overdue present day appreciation of modern mental health in the timely review of their discharge upgrade appeals.”