Spokesmen for the Taleban have rejected an offer of talks from Afghan President Hamid Karzai.
Mullah Omar has been in hiding since the fall of the Taleban
Mr Karzai had said he would be happy for Taleban leader Mullah Mohammad Omar to "get in touch" if he wanted peace.
But Mohammed Hanif, who claims to speak for the Taleban, said Mr Karzai was a "mouthpiece of Americans".
"Such offers are a move to weaken our morale," he told the Associated Press. The Taleban, toppled in late 2001, are fighting to oust the Karzai government.
President Karzai was to blame for the deaths of thousands of Afghans, Mullah Obaidullah Akhund, who served as Taleban defence minister, told Reuters.
In a message to mark the Muslim festival of Eid al-Adha, Mullah Omar promised to step up attacks in 2006.
He said: "There are some leaders in the Islamic world who have damaged the Muslim world more than the US.
"If Muslim nations do not react to these traitors, the result will be victory for the foes of Islam."
President Hamid Karzai told AP on Sunday that he did not expect Mullah Omar to come out of hiding.
He also said the Taleban head would have to account for his actions.
Last May a Taleban spokesman said the mullah would reject any amnesty offer, which had been floated by the head of the nation's reconciliation body.
Mr Karzai said: "We would like all the Afghans, Taleban or non-Taleban, whoever they are, if they want to come back to their country, to participate in the life of this country. It's their home, they're welcome."
He said that would include Mullah Omar.
"We would see what he has to say, of course. But I don't think he will come. He has so much on his hands against Afghanistan. We don't even know as to where he is hiding," Mr Karzai said.
There has been no sight of Mullah Omar since US forces ousted his regime in late 2001.
Last May, the head of Afghanistan's independent peace and reconciliation commission, Sibghatullah Mojaddedi, said Mullah Omar should be accepted back by the government if he renounced arms.
It was not known at the time if the president backed the call but the US made it clear Mullah Omar would have to answer for his actions.