KABUL, Afghanistan -- On a trip to Afghanistan to visit Navy and Marine forces, the secretary of the Navy, Ray Mabus, said Saturday that he expected both branches to reduce their numbers after the Iraq and Afghan wars, and to return to their seafaring roots -- although both will have some new areas of focus.

The Marines increased their numbers by about 27,000 in 2006 and 2007 to meet the demands of fighting the war in Iraq and the increase in troops there.

“The Marines are going to get smaller,” Mr. Mabus said. “And, they are going to get lighter because the equipment they’ve had for these land wars has been heavier and larger.”

Mr. Mabus declined to put an exact number on the Marines’ reduction in the coming years, but he said that rather than being across-the-board personnel cuts, any reductions would be tailored to the service’s needs and then would be achieved primarily through attrition.

There are currently 19,395 Marines in Afghanistan, with a vast majority in southern province of Helmand, according to the military. There is also a substantial Marine presence in Nimruz Province, which has a long border with Iran. The Navy has 4,892 sailors spread throughout the country.

The Navy may reduce somewhat in size, but above all it will go back to its “core competencies,” Mr. Mabus said. It will retain some new specialties, including explosive ordnance disposal, he said.

Mr. Mabus suggested that with increasing delicacy about the presence of American bases in countries from Iraq and Saudi Arabia to Afghanistan, the naval services would become more important because they offered ways to deploy troops that did not rely on land bases.

“Naval forces can do everything they need to do without impinging on anybody else’s sovereignty, without setting foot on land,” Mr. Mabus said, “and they bring everything they need, whether it’s air power with carrier strike groups, whether it’s Marines with all the equipment they need, including air and an amphibious strike groups.

“It will be increasingly important in the years ahead that you can use the same platforms, the same people, the same equipment to do everything from high-end combat, to irregular warfare to humanitarian assistance, disaster relief and engagement and do it all in a self-contained unit, whether it’s a carrier strike group or an amphibious ready-group,” he said.

One area that sounds as if it will not face cuts and could even be augmented is the Navy Seal teams, which are part of both the Naval Command and the Special Operations Command. It was an elite Seal group unofficially called Navy Seal Team 6 that carried out the mission that killed Osama Bin Laden in May.

“You’ll continue to see an emphasis on special forces because of the unique capabilities they bring in combat, in engagement, in training, in cultural things,” he said.

Mr. Mabus is also here looking at some of the efforts to move the Navy and Marines toward renewable energy. During the war in Libya, oil prices increased $30 a barrel, which added $1 billion a year to the fuel bill for the Navy and Marines, he said.

The Marine Corps, in particular, is investing heavily in portable solar blankets to generate the energy that it had been getting from batteries, and is trying out solar panels as well as more efficient generators.

The reason for the focus is that it has become increasingly clear to military planners that the dependence on fossil fuels is the Navy and Marines’ biggest vulnerability -- in cost and mobility, Mr. Mabus said.

Ships are most vulnerable when they are refueling -- the Navy destroyer Cole was bombed by Al Qaeda while it was refueling off the coast of Yemen, for example -- and guarding fuel convoys in Afghanistan has been particularly hazardous for American Marines: one dies for every 50 fuel convoys that come into Afghanistan, he said. “We’re not doing this because it’s the flavor of the month or trendy,” he said of the effort to use alternative energy sources.