Suicide among American soldiers increased again last year and is at a nearly three-decade high, senior U.S. defence officials told the Associated Press on Thursday.

At least 128 soldiers killed themselves in 2008, said two officials, who spoke on condition of anonymity because the data has not been formally released.

The final count likely will be considerably higher because more than a dozen other suspicious deaths are still being investigated and could also turn out to be self-inflicted.

The number of suicides among U.S. soldiers in 2008 was the highest since record-keeping began in 1980. (Image courtesy of U.S. army)

The new figure of more than 128 compares to 115 in 2007 and 102 in 2006 and is the highest since record-keeping began in 1980.

It also calculates to a rate of 20.2 per 100,000 soldiers, which is higher than the adjusted civilian rate for the first time since the Vietnam War, officials said.

The U.S. army planned to announce the figures at a news conference later Thursday.

Officials have said repeatedly that troops are under tremendous and unprecedented stress because of repeated and long tours of duty because of the simultaneous wars in Iraq and Afghanistan.

Yearly increases in suicides have been recorded since 2004, when there were 64, about half the number now. And they've occurred despite increased training, prevention programs, increased psychiatric staff and other army efforts to stem the rise.

Officials are expected to announce additional attempts to help soldiers at the news conference.

When studying individual cases, officials said they found that the most common factors for suicides were soldiers suffering problems with their personal relationships, legal or financial issues and problems on the job.

The new army report follows one earlier this month showing that the marine corps recorded more suicides last year than any year since the U.S.-led invasion of Iraq in March 2003.

That report said 41 marines were possible or confirmed suicides in 2008, or 16.8 per 100,000 troops. The marine rate remained unchanged because the corps is increasing in size, officials said.

Ground forces bear brunt of wars against insurgents

Soldiers and marines have borne the burden of the two wars, which have required more use of ground forces to fight the insurgencies. But the numbers kept by the services only show part of the picture because they do not cover troops who have returned to civilian life.

The U.S. Department of Veterans Affairs tracks the number of suicides among those who have left the military. The VA said there were 144 suicides among the nearly 500,000 service members who left the military from 2002 to 2005 after fighting in at least one of the wars.

The true incidence of suicide among veterans is not known, according to a report last year by the Congressional Research Service. Based on numbers from the Centers for Disease Control and Prevention, the VA estimates that 18 veterans a day, or 6,500 a year, take their own lives, but that number includes vets from all previous wars.

In October, the U.S. army and the National Institute of Mental Health announced a five-year, $50-million US research program into the factors behind soldier suicides and how to prevent them.