The highest-ranking military officer in the United States said Monday that he expects casualties to rise next year as additional U.S. troops deploy to Afghanistan.

Admiral Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, addressed about 700 troops at Fort Campbell, a sprawling army post on the Kentucky-Tennessee border.

"I am sure we will sustain an increase in the level of casualties, and I don't want to be in any way unclear about that," he said.

"This is what happened in Iraq during the surge, and as tragic as it is, to turn this thing around, it will be a part of this surge as well. I expect a tough fight in 2010."

Many soldiers at Fort Campbell already had orders to deploy to Afghanistan before U.S. President Barack Obama unveiled his plan last week to add about 30,000 troops to the 70,000 soldiers already stationed there.

About 1,500 Marines will leave for Afghanistan this month, a Pentagon spokesman said Monday.

Another 6,200 Marines from Camp Lejeune in North Carolina, coupled with 800 from Camp Pendleton in California, will leave for Afghanistan early in 2010.

The U.S. army is also expected to send a training brigade from Fort Drum, N.Y, to Afghanistan in the spring.

U.S. Defence Secretary Robert Gates signed the orders for the deployment last week.

In his Dec. 1 announcement about the troop surge, Obama also said the U.S. would begin withdrawing its forces in July 2011.

A total of 16,000 troops will constitute the first wave of the U.S. build-up in military strength in Afghanistan.

In addition to the 30,000 military personnel supplied by the U.S., about 7,000 soldiers have been pledged by other NATO-member countries and their allies.