The current force of 14,000 is expected to be cut in half, with those troops arriving home by the end of the spring, a second USA official said.
On the U.S. decision of withdrawing troops from Afghanistan, he said the decision will lead to peace in the neighboring country.
Taliban defended their stance not to meet the Kabul administration and said the Taliban negotiators did not allow Afghan officials to enter the meeting room as they are "powerless" and the Taliban were not willing to meet them.
USA special peace envoy Zalmay Khalilzad has met Taliban representatives during at least three separate rounds of direct talks in recent months as part of the effort to encourage negotiations between the Taliban and Kabul.
"America will work with the Afghan government as long as we see determination and progress".
The White House and Pentagon made no immediate comments.
However, the Western-backed government in Kabul has struggled to counter attacks from the Taliban and other militant groups since the withdrawal of most North Atlantic Treaty Organisation combat troops in 2014.
Air Vice Marshal (retd) Shahzad Chaudhry termed the U.S. decision of partial pullout a "much planned move" and a formal assurance to Taliban for its seriousness in Afghanistan's peace. The Taliban has rejected holding talks with the Afghan government, which it often refers to as a "puppet regime".
A significant reduction in the number of US troops in Afghanistan won't impact upon the security of the war-torn country, a spokesman for President Ashraf Ghani said on December 21.
The effort has not been extremely successful.
However, in 2017 he indicated he would keep boots on the ground in the country indefinitely to prevent the country's collapse amid a Taliban resurgence. "Now the reports of [a USA withdrawal] show a weakening stance by the US, which could subsequently undermine [the] Afghan government's position". Trump believes withdrawing troops from Afghanistan would appeal to his base and fulfill a campaign promise.
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The pullout comes as the United States spearheads global efforts to end the war with the Taliban, which was toppled from power in a US-led invasion in 2001.
The Afghan government issued a terse statement saying it was prepared to ensure the security of its country.
Lieutenant General Kenneth McKenzie, nominated to lead the U.S. military's Central Command, warned this month that unless recruiting and training improve, local forces will not overcome the casualty rate.
His decision was made public a mere few hours after he abruptly announced the USA was withdrawing troops from Syria. They carry out near-daily attacks, mainly targeting security forces and government officials. "Peace efforts require strategic patience, tact, and courage - not improvisation without direction".
"There was little doubt that Ambassador Khalilzad was always working with limited time and a zeal of desperation to achieve something before the president pulled the plug", said Daniel Markey, senior research professor at Johns Hopkins University's School of Advanced International Studies.
It maintained that the Taliban's refusal yet again illustrated their carelessness about regional stability, lack of commitment to peace and their intent upon continuity of war.
President Donald Trump has long pushed to pull troops out of Afghanistan, considering the war a lost cause.
President Trump's announcement was not only a shook for his own country's military establishment but also for concerned regional states which are heavily invested in the Afghan conflict.
Taliban spokesman Zabiullah Mujahid would not comment about the USA troop withdrawal when contacted by AFP.
A drawdown also will deal a fresh blow to allies' trust in the United States and give new room to militant groups al Qaeda and Islamic State in which to operate and plot attacks, they said.