Given the "challenging" security situation, the 28-nation alliance was weighing an increase of the personnel of its "Resolute Support" train, assist and advise mission from about 13,000 now, he told the Welt am Sonntag newspaper.

NATO expected to make a decision by June on the possible troop increase and on whether to lengthen the time frame of the deployments which are now renewed annually, he was quoted as saying.

The US general commanding NATO forces in Afghanistan, John Nicholson, in February told Congress he has a "shortfall of a few thousand" troops needed, warning that "we're in a stalemate".

US-led forces have been fighting in Afghanistan for 16 years, making it already America's longest-ever war.

Since NATO's combat mission formally ended in 2014, Taliban attacks have intensified and Afghan military and civilian casualties have risen.

Taliban dressed in Afghan army uniforms last week slaughtered at least 135 young recruits at a northern base, according to official figures.

The US, which is also targeting the Islamic State group's affiliate in Afghanistan, dropped its largest non-nuclear bomb on the jihadist group's hideouts earlier this month, and has also returned Marines to the volatile Helmand province.

Stoltenberg said NATO also wants to increase its role in the fight against international terrorism, especially by training local forces, saying that aside from its Afghanistan and Iraq missions, "we have the potential to do even more".

He said NATO could get more engaged in Jordan and Tunisia, calling both "islands of stability" in an unstable region.