WARSAW(AFP) - NATO agreed Saturday to maintain troop numbers in Afghanistan and reiterated a funding pledge for local security forces through 2020, but officials did not say when the alliance's longest military engagement might end.

Hundreds of thousands of NATO troops have rotated through Afghanistan since 2001, when the United States ousted the Taleban after the September 11 attacks.

"There's no reason to speculate exactly on how long it will continue," NATO Secretary-General Jens Stoltenberg said at an alliance summit in Warsaw. "What we have seen is we are committed, and we are ready to stay."

About 13,000 NATO troops, of which nearly 10,000 are American, are currently stationed in Afghanistan under Operation Resolute Support, to train and assist Afghan security forces.

But the Afghans have struggled to contain a resurgent Taleban and have suffered brutal losses - more than 5,000 local police and troops were killed in 2015 and this year is proving as deadly.

Attacks by the Taleban and other organisations like Al-Qaeda and the Islamic State in Iraq and Syria (ISIS) group have meant NATO - especially key member the United States - has been unable to leave Afghanistan after ending its combat operation at end-2014.

Stoltenberg said NATO will keep thousands of troops in Afghanistan through 2017 under Resolute Support.

He did not provide additional details, but said troop levels would about the same as this year, with "around" 12,000 deployed.

"Afghanistan still faces serious instability and violence so our continued political, military and financial engagement is of great importance," Stoltenberg said.

The 28-nation US-led alliance would look at the situation again next year before deciding future force commitments.

The development comes on the heels of an announcement by US President Barack Obama, who on Wednesday said he was slowing a planned draw down of US troops from Afghanistan.

Obama had previously vowed to slash troop numbers from the current 9,800 to 5,500 by the end of this year, but on Wednesday said the US would now keep 8,400 in the country.

About 6,700 of these American soldiers will work under the Resolute Support banner - with the rest conducting counter-terrorism operations.

The NATO mission in Afghanistan costs about US$5 billion a year (S6.73 billion), with approximately US$3.5 billion coming from the United States.

Stoltenberg said other allies had "nearly" gathered about US$1 billion for next year. The remaining funds are due to come from Afghanistan.

In return for its continued support, NATO is demanding reforms of the Afghan security forces, which are grappling with deeply entrenched corruption and human rights issues.

Afghanistan President Ashraf Ghani met US President Barack Obama in Warsaw on Friday.

Ghani acknowledged he had much work to do in terms of overhauling his country's institutions and implementing reforms, a US administration official said.

"We need to see greater political unity in Afghanistan, we need to see greater military security and greater self-sustainability by the Afghan security forces which these resources will contribute to," the official said, noting that the only solution for the Afghan conflict is political.

However, diplomatic efforts to engage with the Taliban are in disarray.

The United States in May killed the group's leader, Mullah Akhtar Mansour, in a drone strike in Pakistan.