The Taliban brushed aside a US decision to delay withdrawing troops from Afghanistan, saying it would do nothing to save an "unwinnable war" and promising to step up its campaign against the Western-backed government in Kabul.

President Barack Obama's decision to drop plans for a radical reduction in U.S. forces next year was greeted with relief by the administration in Kabul, which had feared being abandoned by its most powerful ally.

Afghan forces, who took over from international troops after NATO ended most combat operations last year, have struggled to contain the spreading insurgency and remain dependent on allies for air and logistical support.

The U.S. will maintain a 9,800-strong force for most of next year, and instead of reducing it to a token presence based around the fortified U.S. embassy in Kabul, will keep 5,500 troops from 2017 for training and counter-terrorism operations.