By Robin Emmott, Arshad Mohammed and Sabine Siebold

BRUSSELS (Reuters) - The NATO allies decided on Tuesday to hold alliance troop levels in Afghanistan steady at about 12,000 next year and launched a campaign to fund the 350,000 Afghan forces it hopes can some day secure the country against Taliban militants.

Fourteen years after the United States first sent troops to Afghanistan, NATO governments have doubts about the ability of its army and police to defend against Taliban fighters, who briefly took over the northern city of Kunduz in September.

As a result, the 28-member Western alliance is abandoning plans to slash its troop levels by the end of this year.

"We are in Afghanistan to prevent that Afghanistan again becomes a safe haven for international terrorists ... that is also in our security interest to make sure that that doesn't happen," NATO Secretary-General Jens Stoltenberg said at a meeting of NATO foreign ministers.

Excluding U.S. counter-terrorism forces, NATO will have about 12,000 troops in Afghanistan for most of next year, made up of about 7,000 U.S. forces and 5,000 from the rest of NATO and its partners such as non-NATO member Georgia.

At Tuesday's meeting, allies also launched a campaign to raise about $3 billion euros to help pay for Afghanistan's state security forces from 2018.

The Afghan security forces budget, funded by the United States and its NATO allies, is agreed up to the end of 2017. NATO wants to announce further funding for the 2018-2020 period at its next leaders summit in July.

"Afghanistan is still one of the poorest countries in the world, so I think this is a good investment we are making," Stoltenberg told a news conference.

As agreed at the NATO summit in Chicago in 2012, non-U.S. NATO allies and partners such as Japan give a total of $1 billion a year in addition to the $4.1-billion that the United States spends on Afghan security forces every year.

'SOBERING' SITUATION

The Taliban's brief takeover of a provincial capital in late September has shaken confidence in the ability of Afghan forces and both the United States and its NATO allies now say events, rather than timetables, must dictate gradual troop reductions.

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U.S. President Barack Obama had aimed to withdraw all but a small U.S. force before leaving office in January 2017, pinning his hopes on training and equipping local forces to contain Taliban militants fighting to return to power.

However, in October he announced he would maintain the U.S. troop presence in Afghanistan at 9,800 through most of 2016, reducing it thereafter to about 5,500 and effectively leaving a decision on a full withdrawal to his successor.

Washington has spent around $65 billion on preparing the fledgling Afghan security forces, while Afghanistan has also received about $100 billion in aid from international donors.

But General Hans-Lothar Domroese, a veteran of Afghanistan who is Germany's second-most senior general in the Western alliance, recently told Reuters that the security situation is "sobering" and "not as stable as we hoped it would be."

As Reuters reported in October, Germany, Turkey and Italy will keep their current deployments, although that is likely to be reviewed again later next year.

Unlike the United States, NATO has never set an end date to its "Resolute Support" training mission in Afghanistan, a non-combat force that also includes troops from some 40 countries, including NATO members, the United States and their allies.

NATO does want to see Afghanistan eventually take care of its own security and has agreed that no later than 2024, Afghanistan must take "full financial responsibility" for its own security forces, according to a 2012 statement.

(Reporting by Robin Emmott; Editing by Richard Balmforth)