'You are not to use electronic communication or even land lines when communicating,' said a leaflet which the groups distributed recently.
The instructions are apparently a response to what are described as 'moderate successes' U.S. troops have achieved in the past few weeks in their fight to flush out rebel cells.
Internet material, mobile messages and phone calls which the rebels use are now the U.S. military's major source of intelligence.
'The U.S. army has carried out successful raids in the light of the tips obtained from the Internet and mobile phones,' said a source close to a major armed group involved in fighting the Americans.
'Therefore we have decided to rely on oral or written messages,' he said.
