"Our courageous Mujahideen (holy warriors) have been engaging in Jihad (holy war) against a cruel and incursive enemy for a noble cause and presenting countless sacrifices on this path every day," Omar said in his message on the eve of Eid-ul-Adha -- largest annual religious festival of the Muslims -- falling on Sunday, reported Xinhua.
The message insisted that NATO-led troops are losing the 10-year-old war.
"Thousands of their troops have been killed while others face permanent disabilities and mental disorder, compelling their people to rise up in opposition and take to the streets.
The American and Western people have no further taste for extending the Afghan war and seeing their soldiers return in coffins. Our enemy is facing an unambiguous defeat," said the message written in English and Pashtu languages.
The one-eyed Omar, who has escaped the US manhunt since his regime was ousted in late 2001, also called upon his countrymen to renounce foreign forces and support Taliban.
"We once again invite the internal opposition to renounce cooperation with the invaders for the sake of their religion and people and call on them to stand shoulder to shoulder with Mujahideen against the foreign occupation," the message added.
However, the Taliban chief urged his fighters to avoid harming civilians during attacks on foreign troops.
"The Mujahideen have to take every step to protect the lives and wealth of ordinary people in accordance with their religious responsibility," Omar said in the message.
"The common folk must also facilitate Mujahideen in averting civilian losses and sufferings. They should avoid moving close to Americans that patrol in villages and countryside and should actively put into practice the precautionary measures announced by Mujahideen so no harm will reach them during the impending attacks on the invader."
Civilian casualties have been on the rise in the conflict-ridden Afghanistan mainly due to suicide and roadside bomb attacks by Taliban militants.
"A total of 1,462 Afghan civilians have been killed in the first half of 2011, up 15 percent over the same period in 2010," the United Nations Assistance Mission in Afghanistan (UNAMA) said in mid-year report released here in July.
The report attributed 80 percent of the civilian deaths in the first six months of this year to the attack of Taliban insurgents and other armed groups opposing the Afghan government.