WASHINGTON - Violence in Afghanistan is on the rise, according to a new Pentagon report to Congress that says the Taliban was emboldened by the reduced U.S. military role and can be expected to build momentum from its 2015 attack strategy.

The number of effective insurgent attacks rose this year, causing increased casualties among Afghan security forces, the report said. While Afghan forces have demonstrated a will to fight and to learn from their battlefield mistakes, the report said the Taliban's resilience has made security fragile in key areas and at risk of deteriorating in others.

Sen. Bob Corker, chairman of the Senate Foreign Relations Committee, told reporters that it was "probably the most depressing assessment" of the direction in which the country is headed that he has heard in a "long, long time."

"We're certainly not in a positive trajectory right now in Afghanistan," Corker said. "In fact, I would say currently, my assessment would be that it's a very negative trajectory."

The Pentagon report, the latest in a regular series of Pentagon war updates required by Congress, also said the Taliban-led insurgency has been emboldened by the U.S. transition from direct combat operations to a train-and-advise role.

"As a result, the Taliban will continue to test the [Afghan forces] aggressively in 2016," it said.

The U.S. now has about 9,800 troops in Afghanistan, some of whom are involved in counterterrorism missions.

In October, President Obama announced that he would keep troops levels steady through most of next year. By the end of 2016, rather than draw down to a Kabul-only U.S. military presence of about 1,000 troops as previously planned, Obama decided the U.S. will maintain 5,500 troops in Kabul and Bagram. Tuesday's report said Obama's decision reflects the need to give Afghans more time to develop a credible army.

"The resilient Taliban-led insurgency remains an enduring threat to U.S., coalition, and Afghan forces, as well as to the Afghan people," the report said.