The statement listed the Taliban's goals, including complete independence of Afghanistan and the establishment of an Islamic system.
"Our first priority is to achieve these goals through talks and negotiation," it said.
"But if the invading powers in Afghanistan are not ready to give the Afghans their natural rights ... then the Mujahideen of the Islamic Emirate are determined to carry on the fight until the realisation of the said goals."
At a conference in London late last month, Karzai called for a "loya jirga" - or assembly of elders - as a start to peace talks and announced an international fund to reward Taliban fighters who disarmed.
But the Taliban statement rejected Western support for Karzai's reconciliation efforts as "an eyewash" designed to convince anti-war voters in the West that their leaders want peace, even as they prepare for a new offensive in Helmand province.
'The largest offensive'
On Thursday, Nato announced that thousands of Afghan and foreign troops are about to launch an offensive in southern Afghanistan.
The operation - the largest the country has seen since the 2001 US-led invasion - is believed to target the Marjah area, a stronghold of around 1,000 Taliban fighters.
Marjah is also thought to be the hub of the Taliban-controlled opium trade - which provides them with most of their funding.
The Afghan president has consistently made overtures to the Taliban, and the West has been increasingly supportive of proposals to lure fighters back into the political process in a bid to end years of fighting.