The US embassy in Kabul urged extreme caution during this period of "heightened threat" but gave no details on potential targets or methods of the planned attack.
"US embassy Kabul has received credible reports of an imminent attack in Kabul... Within the next 48 hours," it said in an emergency warning posted on its website.
"The security situation in Afghanistan is extremely unstable, and the threat to all US citizens in Afghanistan remains critical."
The meeting between Pakistan Prime Minister Nawaz Sharif and Afghan President Ashraf Ghani could be a possible first step towards resuming Islamabad-brokered Taliban peace talks.
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"A meeting between (Sharif) and (Ghani) on the sidelines of the UN climate change conference is likely," a Pakistani official said on condition of anonymity.
He said that Pakistan was ready to facilitate an Afghan-owned and Afghan-led peace process, but there was no immediate reaction from Kabul.
Pakistan, which wields considerable influence over the militants, hosted a historic first round of negotiations in July but the talks stalled soon thereafter when the Taliban confirmed the death of their longtime leader Mullah Omar.
The United States and China have been pushing for the process to restart, but frosty ties between Islamabad and Kabul have been hampering those efforts.
Kabul has turned increasingly bitter over Islamabad's backing for the resurgent Taliban, who have intensified attacks on Afghan government and foreign targets in recent months.