The number of suicides in the US military dropped by 18 percent in 2013 but rose among part-time soldiers in the reserves, the Pentagon said Friday.

A new report said 261 active-duty troops took their lives last year, compared to 318 in 2012, according to "preliminary" figures.

Suicide levels, however, have not dramatically changed over the past six years even though large numbers of US forces are no longer engaged in combat in Afghanistan and have withdrawn from Iraq.

Although the overall number had declined for 2013, suicides increased five percent among those in the US Army National Guard and Reserves, the report said.

Suicides rose to 213 last year among reservists, up from 203 in 2012, it said.

Reservists sometimes lack access to the kind of support services available to active-duty, full-time troops, and it was possible the suicide numbers might reflect that gap, officials said.

US commanders have struggled to stem the suicide problem and have yet to identify its precise causes.

The relentless pace of deployments to Iraq and Afghanistan over the past decade has often been cited as a likely trigger for the suicide problem in the armed forces, but the latest figures failed to support that hypothesis.

When fighting peaked for American troops in Iraq, the number of suicides reached 268 in 2008 and 309 in 2009, figures that are similar to the past two years.

Friday's report said only 13 percent of those who took their lives last year had experienced "direct combat" while 57 percent had deployed to war zones over the past decade.

The "most prevalent" aggravating factors were failed relationships, a history of administrative and legal problems and "financial or workplace difficulties," it said.

The overwhelming majority of those who killed themselves were male, white, under the age of 25, low-ranking enlisted troops who were married, according to the report.

About 65 percent of the suicide victims used a gun to take their lives, but the firearms were bought privately and were not weapons issued by the military, it said.

In 42 percent of all suicides, there was a record of behavioral health problems, and in a majority of cases, the service member had sought assistance from health or other support services within 90 days prior to committing suicide.

In its bid to tackle the problem, the Pentagon has hired more than 9,000 mental health professionals, including psychologists, psychiatrists, nurses, social workers and counselors.

The suicide rate for the whole military was 18 per 100,000 troops last year, compared to 22.7 in 2012. The Army led all branches with the highest suicide rate, followed by the Marine Corps.