US President Barack Obama said Thursday the US would maintain 9,800 troops in Afghanistan “through most of next year” and keep half as many when he leaves office, warning that the security situation is still fragile.

Obama said the plan to keep 5,500 troops in the country when he leaves the White House in 2017 offered the best chance for long-term success.

"It's the right thing to do," he said at the White House. "As commander-in-chief I will not allow Afghanistan to be used as safe haven for terrorists to attack our nation again."

The US president said the Afghan military had made gains but was still not as strong as it needs to be.

Obama had originally planned to pull out all but a small, embassy-based US military presence by the end of next year, a timeline coinciding with the final weeks of his presidency.

But military leaders argued for months that the Afghans needed additional assistance and support from the US to beat back a resurgent Taliban and hold onto gains made over the last 14 years of American bloodshed and billions of dollars in aid.

Obama’s announcement was welcomed by NATO Secretary-General Jens Stoltenberg, who said it “paves the way for a sustained presence by the NATO alliance” in Afghanistan.

"In the coming weeks the alliance will take key decisions on the future scope of the rest of its support mission," Stoltenberg told reporters in Montenegro. "The significant US contribution will be an important factor as we consider the way ahead."

Ghani partnership

US officials had been hinting at the policy shift for weeks, noting that conditions on the ground in Afghanistan have changed since Obama's initial decision on a sharper troop withdrawal timeline was made more than two years ago.

The White House has also been buoyed by having a more reliable partner in Afghan President Ashraf Ghani, who succeeded the mercurial Hamid Karzai last year.

"The narrative that we're leaving Afghanistan is self-defeating," Defence Secretary Ash Carter said Wednesday during a speech at the Association of the US Army. "We're not, we can't and to do so would not be to take advantage of the success we've had to date."

While officials said the Afghan policy had been under review for several months, Obama's decision to leave more forces in Afghanistan than initially envisioned was reinforced when Taliban fighters took control of the key northern city of Kunduz late last month, prompting a protracted battle with Afghan forces on the ground, supported by US airstrikes.

During the fighting, a US airstrike hit a hospital, killing 22 people, including 12 Doctors Without Borders staff and 10 patients.

IS threat

Beyond the recent security troubles in Afghanistan, US commanders have also expressed concern about members of the Islamic State (IS) group moving into the country and gaining recruits from within the Taliban.

The troops staying in Afghanistan beyond next year will continue to focus on counter-terrorism missions and training and advising Afghan security forces, the officials said. They will be based in Kabul and Bagram Air Field, as well as bases in Jalalabad and Kandahar.

The president's decision to keep the US military in Afghanistan beyond his tenure thrusts the conflict into the 2016 presidential race. The next president will become the third US commander-in-chief to oversee the war, with the options of trying to bring it to a close, maintaining the presence as Obama left it or even ramping up US involvement in the conflict.

Until now, Afghanistan has barely featured in campaign discussions on foreign policy and was not mentioned in Tuesday's Democratic debate. The war was discussed only briefly in two Republican debates.

(FRANCE 24 with AP)