Omar claimed that young Afghans are enlisting in large numbers to become suicide bombers in a spring offensive that will target U.S. and other foreign troops in Afghanistan.
The statement lends support to warnings by U.S. and Afghan defense officials, who have warned in recent months that the Taliban and Al-Qaeda are planning to step up attacks in Afghanistan.
Omar's message, which was delivered by telephone to news agencies in Afghanistan and Pakistan, could not be authenticated.
The statement came on the same day that police reported the arrest of two Taliban suspects carrying letters from Mullah Omar and Al-Qaeda commander Ayman al-Zawahri.
Police say the arrests were made separately in the eastern province of Nangarhar, near the border with Pakistan.