Soldiers of Afghanistan's National Army walk beside their vehicles on the outskirts of the city of Lashkar Gah on Oct. 18. A senior U.S. commander said Afghan military casualties are “unsustainable” ahead of coalition forces’ departure. (Abdul Khaliq/AP)

Afghan forces are dying at an unsustainable rate on the battlefield, a senior U.S. commander said Wednesday , in a troubling sign ahead of the departure of most U.S. and NATO troops by the end of this year.

Lt. Gen. Joseph Anderson, a senior commander for U.S. and NATO forces in Afghanistan, said 4,634 members of Afghanistan’s police and army have been killed in action this year. That is already an increase from the total for 2013, when 4,350 members died in the line of duty.

“This is not sustainable,” he told reporters in a video briefing from Afghanistan.

Anderson attributed much of that rise to the intensified combat that Afghan forces have faced since they took full responsibility for security from foreign troops in mid-2013. That shift has put the country’s relatively inexperienced military more directly in the line of fire against a resilient Taliban insurgency.

“We expected that [rate] actually to be much higher based on the role they’ve played and where they’ve been,” Anderson said.

He said Afghanistan was taking steps to reduce troop deaths, including working to transport wounded soldiers more efficiently to medical facilities and teaching police to use equipment to protect themselves from roadside bombs.

“They do need to decrease their casualty rate,” he said.

The combined size of the Afghan army and police forces, a total of 352,000, takes on new importance as U.S. and NATO soldiers wind down the international mission launched after the attacks of Sept. 11, 2001.

While the Obama administration has dealt serious blows to al-Qaeda in Afghanistan and Pakistan, the Taliban remains a potent enemy, capable of planning attacks from across Afghanistan’s border with Pakistan.

Under a plan announced by President Obama, the U.S. force in Afghanistan, now about 20,000, will shrink to 9,800 by Jan. 1. Those troops, alongside a small contingent from allied nations, will continue to advise Afghan forces and conduct targeted operations against remaining al-Qaeda fighters in the region.

By 2017, the U.S. force will be reduced even further and will be based at the U.S. Embassy in Kabul.

Anderson said Afghan forces had proved themselves over the past year, heading off attacks timed around Afghan elections this spring and summer and fighting back Taliban attempts to reclaim territory.

Ronald E. Neumann, a former U.S. ambassador to Afghanistan, said the Taliban’s objectives have changed from what they were from 2010 to 2012, when militants fought intense battles with U.S. forces during Obama’s troop surge in Afghanistan.

Today, he said, “you’ve got a heavy effort by the Taliban to break the Afghan forces.”

Taliban attacks have also grown more deadly for Afghan civilians. In July, the United Nations said civilian deaths had increased by almost a quarter during the first half of 2014 compared with the same period in 2013.

David S. Sedney, a former senior Pentagon official, warned that Afghan forces are likely to continue to struggle in confrontations with the Taliban as the United States accelerates its withdrawal in the final months of 2014 and as Afghans find themselves increasingly without assistance in key areas such as intelligence and air support.

“Now the Taliban is stepping into that vacuum,” he said.