WASHINGTON -- The military commander who directed the raid that killed Usama bin Laden is warning that the escalating demands on U.S. special operations forces are hampering their training and could slowly eat away at their combat readiness.

Vice Adm. William McRaven said demand for the elite forces around the world continues to grow, so there often isn't enough time to train between deployments. And he said the helicopters and other equipment they need are not available to units in the United States who are preparing to deploy.

Special operations forces "cannot indefinitely sustain current levels of overseas presence," said McRaven, who has been nominated to replace Adm. Eric Olson as commander of the U.S. Special Operations Command. "The resulting pressure on the force and our families is too great, and the pressure is creating a dramatic effect on our readiness."

He said the short breaks between deployments limit training in key language skills and the regional and cultural expertise that enable the commandos to work well in other countries. And he noted that most of the helicopters needed for training are either at the warfront or in maintenance, making it difficult for aircrews to hone their skills.

The lack of helicopters, aircraft and ships at bases in the U.S., he said, limits training on refueling, live bomb drops or dock landings.

McRaven's comments came in answer to questions from the Senate Armed Services Committee during a hearing Tuesday and in a written questionnaire obtained by The Associated Press. And they mirror, in part, observations made by Olson earlier this year, when he warned that the elite forces were "beginning to show some fraying around the edges."

McRaven, a career-long special operator, was serving as head of the Joint Special Operations Command in Afghanistan earlier this year and was tapped to be the operational commander of the Navy SEAL team raid into Pakistan last month that killed bin Laden.

While McRaven said trends show that special operators are more resilient and able to handle the stress, steps must be taken to ease the strain.

Senators pressed McRaven on the impact that the planned U.S. troop withdrawal from Afghanistan would have on special operations troops, asking whether Afghan elite forces would be able to step in.

McRaven said that right now U.S. forces need to continue to monitor and guide many of the Afghan special forces, but some units are highly trained and are increasingly taking on a larger role.

While the number of special operations forces has doubled to about 61,000 over the past nine years, the total of those deployed overseas has quadrupled. There are at least 7,000 special operators in Afghanistan and about 3,000 in Iraq. Those numbers can vary as units move in and out of the war zone, and often the totals don't include the most elite of the commandos -- special mission units such as Army Delta Force and Navy SEALs that may go in and out more quietly and quickly.

Time spent at home between deployments can vary depending on missions or assignments. But in the worst cases, McRaven said that for every 100 days a special operator is out in the field, he or she spends just 80 days at home. In other, better cases, they are deployed for 100 days and can get up to 200 days at home.

One of the key pressures on the force has been the lack of predictable deployments. Often, McRaven said, there are last-minute shifts in schedules or changes in mission requirements. At the same time, the intense focus on deployments to Iraq, Afghanistan, Pakistan and the surrounding region have made it difficult to expand the use of commandos in other hot spots.

In Afghanistan, special operations forces serve a number of roles. Not only do they mount an aggressive counterterrorism campaign across the country, but they also form teams to train or mentor Afghan forces. In one example, McRaven said that over the past 12 months, the task force he commanded conducted about 2,000 operations, roughly 88 percent of which were at night.

The heavy demands in Afghanistan and Iraq make it difficult to meet the needs of other commanders in hot spots around the world. And part of that is because special operations forces rely on regular forces -- often the Army -- to provide support, logistics, intelligence and surveillance, including unmanned drones.

McRaven said Special Operations Command is working on a number of programs to ease the stress on the force and provide the training they need. He said commanders will set maximum deployment rates for each element of the force, provide greater predictability and set up more opportunities to train closer to home when they are not overseas.

The military, he said, also has increased pay for language skills, and is using contracts to get aircraft that can be used for training.

In other comments to the Senate committee, McRaven said that battling terrorism threats in Africa currently is taking a back seat to the conflicts in Iraq and Afghanistan, and most of the surveillance assets are in the war zones. As U.S. troops pull out of those two countries, there may be more opportunities to address insurgent threats in Africa.

He said other restrictions hamper operations in Africa, including limited communications and lack of bases and international agreements on flight paths.

Since many countries there also balk at having any significant U.S. force within their borders, McRaven said one solution would be to substantially boost the use of sea-based intelligence, surveillance and reconnaissance capabilities.