KABUL, Afghanistan —  Militants based in Pakistan are planning to infiltrate relief organizations and companies in Afghanistan as part of a plot to abduct U.S. citizens, the American Embassy warned Wednesday.

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 nongovernment 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 (search) 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 homicide 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.