Stoltenberg's remarks in Kabul came hours after the Taliban stormed a remote army post in the country's west, which local officials said had killed at least 20 soldiers.

Another 20 troops were missing after the overnight raid in Farah province's Pusht Koh district, provincial council member Dadullah Qaneh told AFP, as Afghan forces struggle to beat back the insurgents across the country.

The Taliban claimed responsibility for the attack, saying more than 20 soldiers were killed and five captured.

Despite the escalating violence, Stoltenberg struck a relatively optimistic tone during his unannounced visit to the Afghan capital.

"The potential for peace is greater now than it has been in many years," the NATO secretary general told a joint news conference with Afghan President Ashraf Ghani.

But he acknowledged "the situation remains serious".

"The Taliban must understand that continuing the fight is pointless and counterproductive," he said.

"We need an Afghan-owned and led peace process. And it must be inclusive."

Ghani thanked the alliance for its support of Afghan troops, which have been "bearing the burden" of the conflict since the withdrawal of US-led NATO combat soldiers at the end of 2014.

NATO's Resolute Support mission in Afghanistan is to train and assist local forces. It has around 16,000 troops in the country, the majority of them American.

Stoltenberg's visit comes after a spate of insider attacks by Afghan soldiers that have killed or wounded several NATO servicemen.

- Weak security -

The Taliban has been ratcheting up pressure on Afghan police and soldiers this year, even as it shows a tentative willingness to hold bilateral talks with the United States in Qatar.

A recent US government watchdog report said Kabul's control of Afghanistan had slipped in recent months as local forces made little or no progress against the Taliban.

The US Special Inspector General for Afghanistan Reconstruction (SIGAR) cited Resolute Support as saying this summer's casualty toll for Afghan forces has been worse than ever.

"From the period of May 1 to the most current data as of October 1, 2018, the average number of casualties the (Afghan forces) suffered is the greatest it has ever been during like periods," Resolute Support said, according to SIGAR.

Underscoring the security weaknesses, powerful police chief General Abdul Raziq was among three people killed in a brazen insider attack on a high-level security meeting last month in Kandahar that was claimed by the Taliban.

Stoltenberg's visit comes ahead of an international gathering in Moscow on November 9 that aims to kickstart peace talks between Kabul and the Taliban.

There are concerns the Russian meeting, which an Afghan delegation has agreed to attend, could derail recent efforts by US peace envoy Zalmay Khalilzad to get the militants to agree to end the war.