At a base near the eastern city of Jalalabad, Hagel said it was crucial the United States and its partners ensure hard-fought progress did not unravel as the US-led force draws down to about 12,500 troops.
"There are challenges ahead. The job's not over. This is still a dangerous country in many ways," he told a gathering of US and Polish soldiers.
"We don't want to see that tremendous progress that's been accomplished... We don't want to see that roll back downhill," he said.
The training ranges from instruction on firing artillery to press relations and countering roadside bombs, officers told reporters.
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Hagel said the outpost will serve as "a model" for the mission next year, with several other Train, Advise, Assist Command (TAAC) bases being set up across the country.
The goal was to strengthen the Afghan army and police in time for the complete withdrawal of US and coalition troops in 2016, he said.
Washington had planned to deploy a maximum of 9,800 soldiers in 2015 but an additional 1,000 US troops will remain in Afghanistan next year to make up for a temporary shortfall in NATO forces, Hagel announced on Saturday in Kabul.
At the peak of the foreign military intervention in Afghanistan four years ago, about 130,000 NATO troops were deployed, following the 2001 fall of the Taliban regime that sheltered Al-Qaeda.
Despite a spate of Taliban attacks in Kabul and record casualties this year among Afghan forces, Hagel and top commanders voiced cautious optimism about the country's prospects under newly-elected President Ashraf Ghani.
"It's like a night and day change working with this government, " the general said. "You have a president that has embraced the international coalition, has embraced the Afghan security forces.