Under current plans, the US will cut its troop numbers from 9,800 to 5,500 by the end of 2016. But in the face of a resurgent Taliban, former commanders have urged President Barack Obama to keep 9,800 there into next year.
The diplomat said it appears that at the NATO summit in Warsaw in July, the allies will shelve earlier plans to consolidate forces in and around Kabul next year.
In addition, the diplomat said officials believe NATO will get commitments for the USD 5 billion needed to fund the current number of Afghan security forces through 2020.
The diplomat was not authorised to discuss the expected decisions publicly so spoke on condition of anonymity.
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Asked about the plans, British defense secretary Michael Hammond told reporters that, "Nobody wants to see all the spokes collapse. I think we're all aware of the fragility of the Afghan forces. They're fighting hard, but taking very heavy casualties."
The expected decisions cement NATO's continued commitment to the training and advising mission in Afghanistan, as the conflict there drags on.
And they come on the heels of an announcement by President Barack Obama to expand the US military's authority to support the Afghan forces in the air and on the ground.
The US is now able to conduct airstrikes against the Taliban when needed in critical operations, and American troops can accompany and advise Afghan conventional forces on the ground, much like they have done with Afghan commandos.
Afghan forces are still struggling against a stubborn Taliban, which has made gains this year, particularly in the south. And Islamic State fighters have also sought to gain a foothold in the country.
The diplomat said there have been no changes yet to the decision to reduce forces, and it will be possible to continue working with the Afghans in Kabul and all four of the regions with the lower US troop levels.
But, the diplomat also noted that Obama has shown a willingness in the past to adjust troops cuts when conditions on the ground call for it.
So far, the diplomat said that the U.S. Has not asked other allies to increase their troop commitments to make up for the US reduction.