WASHINGTON — The Pentagon has pushed an increasing percentage of its troops to combat tours in Iraq and Afghanistan in the past two years, seeking to spread the burden on forces strained by multiple deployments, records show.

Through June, 57 percent of active-duty soldiers, Marines, airmen, sailors and Coast Guardsmen have served in or near Iraq and Afghanistan. That's up from 50 percent in August 2006.

The Army, which shoulders most of the combat, has shifted many soldiers to specialties needed for the fight. They include infantry, military police and intelligence. In 2006, 58 percent of active-duty soldiers had served combat tours. That compares with 68 percent in 2008. About 10 percent more are in initial training and will soon be eligible for a combat deployment, said Lt. Col. Anne Edgecomb, an Army spokeswoman. Soldiers who haven't served in Iraq or Afghanistan may have medical problems, or they have specialties such as foreign language skills useful in other parts of the world, she said.

The percentage of soldiers who have served multiple deployments has jumped as well. Today, 31 percent of soldiers have been to war zones more than once. That compares with 20 percent in 2006. The number of soldiers with more than five tours has increased to 2,358 in 2008 compared with 961 in 2006.

Top officials, including Adm. Michael Mullen, the chairman of the Joint Chiefs of Staff, have expressed concerns about the stress of multiple tours on troops and their families.

In June, Mullen told soldiers at Fort Stewart in Georgia that he worried they'd leave once their obligations had expired. "We can't afford to lose you," he said. "There's a finite amount of gas left in this tank. And in my view we've used at least half the tank."

The Army recently reduced its standard combat deployments from 15 months to 12. Marine tours generally last seven months. Air Force tours are usually four or six months.

"Some of our soldiers have deployed more frequently than others because of their unique skills and the high demand for those skills in Afghanistan and Iraq," Edgecomb said. "But in a relatively short amount of time, we've made progress in adjusting the structure of our force to better meet current operational needs, which means more troops share the burden of deployment."

The effect of multiple tours depends "on how hot the conflict is," said James Martin, a retired Army colonel and expert on military culture at Bryn Mawr College in Pennsylvania.

Martin said commanders should carefully monitor soldiers and Marines who face the most stressful combat assignments, calling them "canaries in the coal mine." He added, "Those who are most exposed and in the most challenging spots are at greater risk for post-traumatic stress," he said.