The embassy said the warning, which also cautioned that Americans were at risk from suicide attacks, was based on fresh intelligence and urged Americans to take stringent security precautions.
The Pakistani militants "planned to seek employment with non-government organizations or present themselves as construction contractors to gain access to the organizations prior to conducting a kidnapping operation," it said in a note circulated to U.S. citizens.
The note didn't identify the militant group or the organizations it might target in Afghanistan. It said it was unclear when any attack might take place.
The note also urged foreigners to be on guard for rocket attacks, assassinations, hijackings and shootings.
The warning comes a week after kidnappers released three U.N. workers — one of British-Irish nationality, one from Kosovo and one from the Philippines — who were held captive for nearly a month. All were unharmed, but the kidnapping sent a chill through Kabul's expatriate community and stirred concern that Afghan militants were copying the tactics of their counterparts in Iraq.
Foreigners in the capital were already on edge after a suicide attack in a popular Kabul shopping street on Oct. 23. An American woman was killed in the blast, although NATO peacekeepers buying carpets appeared to be the target.
Elsewhere, the bodies of six Americans who died when their plane crashed high in Afghanistan's snow-covered mountains have been recovered, U.S. military officials said Wednesday.
The plane went down Saturday, but search efforts were complicated by bad weather and difficult terrain, said military spokesman Maj. Mark McCann.
"We regret to report that all six individuals on board the aircraft – the three U.S. civilian crew members and three U.S. soldiers – were killed in the crash,'' McCann said.
He said the victims' identities would be released later by the Defense Department and Florida-based Presidential Airways, which had contracted the CASA 212 transport plane to the U.S. Air Force.
A plane carrying the bodies of the six victims back to the United States left Bagram on Tuesday evening, bound for Dover Air Force Base in Delaware, McCann said.
"An investigation will be conducted to determine the cause of the crash. However, at this time, we have no indication this crash was caused by hostile fire," McCann said.
Accidents have accounted for most of the more than 100 deaths of U.S. service members since Operation Enduring Freedom began in the wake of the Sept. 11, 2001, attacks on the United States.
McCann said the bodies of the six Americans will be returned to their families "as soon as possible."