Pentagon chief Chuck Hagel on Sunday told US troops in Afghanistan their mission was not over as they shift to a scaled-back role training their Afghan counterparts.

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.

In his last visit to Afghanistan as defence secretary after resigning last month, Hagel flew to Gamberi base to get a first hand look at preparations for a new, much smaller NATO mission that will begin in the new year.

The training ranges from instruction on firing artillery to press relations and countering roadside bombs, officers told reporters.

- Preparing for 2016 exit -

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.

"We'll be working ourselves out of a job," said Hagel, who later flew on to Kuwait for the next leg of his overseas trip.

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.

"I've seen a huge improvement after the inauguration (of the new president)," General John Campbell, commander of the NATO-led force, told reporters on Saturday.

"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."

US relations with Kabul were severely strained under former president Hamid Karzai, who often blasted American troops and diplomats.

Additional 1,000 US troops to stay in Afghanistan: Hagel
Kabul (AFP) Dec 06, 2014 - An additional 1,000 US troops will remain in Afghanistan next year to meet a temporary shortfall in NATO forces, US Defense Secretary Chuck Hagel said Saturday during a visit to Kabul.

President Barack Obama approved the move despite an earlier plan to limit the US force to a maximum of 9,800 troops in 2015.

A protracted Afghan election delayed the signing of security deals with the United States and NATO countries, which set back plans for Western governments to contribute troops to the post-2014 mission, named "Operation Resolute Support", Hagel said.

Due to "delays in signing these agreements, the force generation effort for (Operation) Resolute Support is several months behind where we hoped it would be at this time", Hagel told reporters.

"As a result, President Obama has provided US military commanders the flexibility to manage any temporary force shortfall that we might experience for a few months as we allow for coalition troops to arrive in theatre."

"This will mean the delayed withdrawal of up to 1,000 US troops -- so that up to 10,800 troops, rather than 9,800, could remain in Afghanistan through the end of this year, and for the first few months in 2015."

Hagel said Obama's decision did not change the mission for the troops next year -- which will focus on training Afghan forces -- nor did it alter a long-term deadline for a US troop drawdown over the next two years.

The Pentagon chief also said US forces would maintain "a limited counter-terrorism mission" against Al-Qaeda militants in Afghanistan.

Washington was "committed to preventing Al-Qaeda from using Afghanistan as a safe haven to threaten the United States," he said.