Afghan security forces are being killed or wounded in attacks by the Taliban and other groups in even higher numbers than last year, when they suffered devastating losses, a NATO official said Thursday.

Authorities estimate about 5,000 local police and troops were killed in 2015 -- with an additional 15,000 wounded.

The grim numbers are set to increase this year, said US Brigadier General Charles Cleveland, a senior spokesman for NATO's Operation Resolute Support, which helps train and advise Afghan partners.

"What we do know is that the pace of casualties for the Afghans this year has been higher," Kabul-based Cleveland told reporters in a video call.

"We are concerned about the number of Afghan casualties, and we work with the Afghans as much as we can to try and reduce those."

He did not provide numbers, but a subsequent NATO email said that the increase was about 20 percent so far this year from the comparable period in 2015.

The substantial losses highlight Afghanistan's ongoing security crisis, which has worsened in recent months with frequent Taliban offensives and attacks by other groups.

Afghan forces at the start of 2015 succeeded NATO in handling most of the country's security, but despite extensive training and assistance from the alliance, they are struggling to contain the Taliban.

On Wednesday, militants stormed the American University of Afghanistan in Kabul in an unclaimed assault that killed 16.

And earlier in the week, with fighting intensifying, NATO announced the deployment of around 100 US troops to Lashkar Gah, the capital of Helmand province, to help head off a potential Taliban takeover of the city.

Cleveland said the high level of casualties actually demonstrated Afghans' resilience.

"For many militaries, that would break their back," he said.

But "they were able to continue to regroup and they were now of course able to move out on the offense."

US President Barack Obama, elected eight years ago partly on a promise to end America's longest war, last month put the brakes on the US pullout from Afghanistan.

He ordered 8,400 troops to remain into next year to tackle the Taliban threat. He had previously vowed to slash the US troop presence from its current level of 9,800 to 5,500 by 2017.

The Afghans have about 320,000 police and troops. The security forces are grappling not just with attacks but with internal issues such as deeply entrenched corruption.

Nonetheless, Cleveland said, "We still do believe that the ANDSF (Afghan National Defense and Security Forces) is performing better this year than they performed last year."

"We still believe that they are generally on a positive trajectory."