By James Mackenzie and Jibran Ahmad

KABUL (Reuters) - The Taliban on Friday brushed aside a U.S. 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.

The Taliban, which showed its growing strength last month by seizing the key northern city of Kunduz and holding it for three days, said the forces would not be enough to halt its advance.

"Maintaining American troops in Afghanistan can in no way slow down the rapid process of our Jihad and struggle," the Islamist militant group said in a statement that promised further attacks on U.S. troops and installations.

"If the invaders lost the war in Afghanistan with the presence of hundreds of thousands of troops, their hopes of reversing the tide with five thousand troops are also misguided," it added.

U.S. fears over the growing influence of the ultra-radical Islamic State in Afghanistan, as well as concerns that denying support to President Ashraf Ghani would risk the rapid collapse of his government, prompted Obama to drop a promise to pull troops out before his presidency ends in Jan. 2017.

But there was no word on what political solution may be possible to end a 14-year conflict.

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One militant commander close to the Taliban's leader Mullah Akhtar Mansour said Obama's move destroyed any hope of a revival in the stalled peace process.

"It means they aren't sincere about a peaceful solution to the Afghan crisis," he said, speaking on condition of anonymity.

"It's a joke. They will have to leave Afghanistan, but they will damage the peace process through these tactics," he said.

Thomas Ruttig, co-director of the Afghanistan Analysts Network, said Afghan troops needed more support, but he added: "The war cannot be won, otherwise 140,000 U.S. and other troops should have won it. Conclusion? The solution must be political."

"THEY SHOULD STAY"

Ghani's national unity government with Chief Executive Abdullah Abdullah has come under growing strain in the crisis, particularly in the wake of the fall of Kunduz and ongoing attacks in the capital Kabul.

For some Afghans, Obama's move was welcome.

"As the war continues all over Afghanistan, the presence of U.S. troops is hugely needed and they should stay here," said Kabul resident Haji Fahim.

But there was little optimism among Western analysts and diplomats that the troop contingent would do much to reverse a deteriorating situation, given the lack of a peace process.

Without talks involving neighboring powers including Pakistan, blamed by many Afghans for backing the insurgency, and a sustained assault on corruption which has blighted government, there would be no lasting stability, said Afzal Ashraf, a consultant fellow at the Royal United Services Institute in London.

Afghan security forces, theoretically numbering almost 350,000, have suffered heavy combat losses and desertions and still need their international allies.

U.S. military support for the Afghan army has been in the spotlight following a deadly U.S. air strike on a hospital in Kunduz run by medical charity Medecins Sans Frontieres, prompting calls for tighter controls over such attacks.

The Taliban's ability to strike outside its main heartlands in the south and east of the country was amply demonstrated by its success in Kunduz, a region where it has traditionally not been strong.

It has also advanced in other areas, cutting off the main highway linking Kabul with the key southern city of Kandahar and threatening the provincial center of Ghazni this week.

(Additional reporting by Krista Mahr and Sayed Hassib; Editing by Mike Collett-White)