Kabul welcomed on Friday the US decision to keep thousands of troops in Afghanistan past 2016, vowing to respond to a resurgent Taliban "with full force" even.

Kabul welcomed on Friday the US decision to keep thousands of troops in Afghanistan past 2016, vowing to respond to a resurgent Taliban "with full force" even as the armed groups promised to wage jihad until the last American soldier leaves.

President Barack Obama's decision Thursday to keep a 9,800-strong US force in Afghanistan through much of next year came as he admitted that Afghan forces were not ready to stand alone.

Calling the plan "the right thing to do", Obama acknowledged "Afghan forces are still not as strong as they need to be".

"As commander in chief, I will not allow Afghanistan to be used as a safe haven for terrorists to attack our nation again," he said in a televised address.

Afghanistan welcomed the decision, saying in a statement late Thursday it "will respond to fear and terror with full force", but adding that it will keep "ajar the door to peace".

President Ashraf Ghani also tweeted his support for the decision.

"Beside stability/development of AFG, we remain determined to strengthen the relations in the area of fighting terrorism now more than ever," he wrote.

Recent intense fighting has underscored the continued role of US troops in training the Afghan forces.