Omar in an address marking the end of the holy month of Ramadan said victory over NATO and U.S. forces was in the hands of the Afghan fighters.
"Put all your strength and planning behind the task of driving away the invaders and regaining independence of the country," London's Telegraph newspaper quoted his statement as saying.
There are 150,000 U.S. and NATO forces fighting a war in Afghanistan aimed at suppressing an insurgency that grew out of the toppled Taliban regime.
They are fighting a counterinsurgency strategy that formed the basis of a new war plan outlined by U.S. President Barack Obama in December.
"Those military experts who have framed strategies of the invasion of Afghanistan or are now engaged in hammering out new strategies, admit themselves that all their strategies are nothing but a complete failure," the Telegraph quoted the Pashtun-language statement from Omar as reading.
The Afghan war strategy is focused on Taliban strongholds in the south of the country. Obama said he would pull his forces out of Afghanistan by July.
"What we want to convey to you through this message is that (you) withdraw your soldiers from our country unconditionally and as soon as possible," warned Omar. "This is in your interest and in the interest of your people and the best option for regional stability."