Marking Islam's holiest day with a message to his followers, Mullah Omar warned that the Taliban's campaign would become "more forceful and organized" in the coming months, and would eventually drive the NATO-led coalition from Afghanistan.
"Do not be deceived by the enemy's hollow and unworthy propaganda," he wrote in his Eid ul-Fitr message, explaining that American forces in Afghanistan had already "been faced with defeat."
NATO took the reigns of the Afghan coalition, formerly led by the U.S. military, in early October. This year has seen the bloodiest fighting since the Taliban was toppled in 2001, with thousands dead. NATO troops from Canada and the United Kingdom, fighting in the south, have taken on the worst of the insurgency.
"Today, NATO members are allowing their soldiers to be killed in Afghanistan just for the sake of the United States," Mullah Omar wrote. "It is necessary for these NATO member countries to think about their own interests and get out of Afghanistan."
Omar's message was sent to Taliban fighters marking Eid ul-Fitr, the annual feast that closes the holy month of fasting known as Ramadan. The text of the message was e-mailed to Afghan news organizations from an unknown location.
In the message, Omar pressed his "holy warriors" not to inflict unnecessary civilian casualties and not to bicker among themselves, warning that in-fighting weakened the mujahedeen and slowed the campaign to drive out the Soviet Union in the 1980s.
Omar called Afghan President Hamid Karzai a stooge of the United States, stoutly denied claims that his movement is backed by Pakistan, and dubbed the United Nations an "ineffective" and useless organization.
The message gave no detail on the nature of the attacks to come, only warning that the world would see a "surprising level" of fighting from his army.
In a recent telephone statement to ABC News, Taliban commander Mullah Dadullah Akhund claimed he had 500 suicide bombers ready to be launched, and a fighting force of more than 12,000 men.