Immediately after the determination was made public, the US embassy in Kathmandu issued a new security warning, advising Americans in Nepal to keep a low profile and exercise special caution "during the upcoming time period."

"This announcement is issued to alert American citizens in Nepal concerning the possibility of an increased threat to Americans and American-affiliated organizations from Maoist insurgents in the coming days," the embassy said in a notice to US citizens.

The notice did not say why the threat might be heightened but a copy of it was provided to AFP in Washington by the State Department shortly after the publication of Friday's Federal Register, which contained the determination about the Maoists.

"I hereby determine that the Communist Party of Nepal (Maoist)... has committed, or poses a significant risk of committing, acts of terrorism that threaten the security of US nationals or the national security, foreign policy, or economy of the United States," Deputy Secretary of State Richard Armitage said in the notice.

The determination was made the designation amid a spike in fighting between Nepalese troops and Maoist rebels, who want to overthrow the country's constitutional monarchy.