United States (US) military troops in Afghanistan have started working with smaller Afghan units to prepare them for a more aggressive offensive against the Taliban next year in a push to break the stalemate in the 16-year-old war, the top US commander for the Middle East said Thursday.

While acknowledging there is still much more to be done, Gen Joseph Votel sounded optimistic, saying he is seeing some positive trends in the Afghan fight.

As more older Afghan commanders leave or are pushed out of their posts, younger leaders are taking over, he said, adding that the forces are conducting more operations and going on the offensive more often. As a result, he said, officials are seeing the number of casualties start to go down.

"I think we're still very keen to break the stalemate and that's what this effort is about here," Votel told reporters at his US Central Command headquarters.

"I'm not declaring victory here with this ─ but I think some of the steps we've taken... are positive steps that are moving us in that direction to break the stalemate."

Gen Joseph Dunford, chairman of the Joint Chiefs of Staff, told Congress last week that he still considers the war a stalemate.

But he and Defense Secretary Jim Mattis assured lawmakers that the plan to increase US forces in Afghanistan and beef up military support to the Afghan units will pay off.

Congress members, however, have expressed skepticism and frustration with the Pentagon, and complained that they haven't gotten enough information on the administration's new strategy for winning the war in Afghanistan and bringing greater stability to the broader region.

US President Donald Trump in August approved a Pentagon plan to deploy as many as 3,800 additional US forces to Afghanistan, where there are already more than 11,000 serving.

The additional American forces will be used to increase efforts to advise and assist Afghanistan's forces, including putting advisers with smaller Afghan battalions, which they call Kandaks.

Doing so puts American troops closer to the fight, but military leaders say it will allow them to better help the Afghans improve their ability to fight insurgents.

Votel said the advisers will help those Afghan units get ready for next year's fighting season.

The US troops would also be used to beef up US counterterrorism operations against Al Qaeda and a growing militant Islamic State affiliate in Afghanistan, as well as the Taliban and other militant groups.

The added American troops have already started moving into Afghanistan, including a significant number of Army soldiers and some Air Force personnel who went in with an extra six F-16 fighter jets.

The Pentagon, however, has repeatedly refused to even provide estimates of how many of the additional troops have deployed, despite promises that the department will be more transparent with the American people about how many US service members are serving there, in harm's way.

Mattis told reporters traveling with him on Wednesday that more than a dozen Nato allies have agreed to boost their commitments to Afghanistan, although some may just be a symbolic increase.

The Taliban, meanwhile, continues to be a resilient enemy, launching a series of high profile attacks including a recent rocket assault at the airport in Kabul while Mattis was on the ground in the country.

Mattis and other senior leaders say they need to increase the military effort in the country in order to force the Taliban to the negotiating table where they can get a political resolution to the war.

On Thursday, Votel said he is hopeful and believes that peace talks are possible.