Mullah Dadullah, a Taliban commander, said on Tuesday that there would be more attacks on Nato and US forces this year.
Dadullah told Reuters by satellite telephone: "Guerrilla attacks on Nato, American and coalition forces will continue and increase this year. The Taliban will inflict heavy casualties on them."
Dadullah did not comment on the death last month of Akhtar Mohammad Osmani, the most senior Taliban commander to be killed by US forces since 2001.
Osmani was killed in a US air raid in the south, and another rebel commander, who declined to be identified, said earlier his death would be a blow to the Taliban.
About 40,000 foreign troops are in Afghanistan, some 32,000 of them under Nato command. They are trying to ensure enough security to enable development projects to get started.
Dadullah said the Taliban had used a winter lull in fighting to draw up new war plans to inflict maximum damage on foreign forces. Afghan fighting traditionally falls off during the winter when snow blocks mountain passes.
Dadullah said: "They will soon come to know about the Taliban's strength and war strategy. We will attack with such a force they will have no time to settle."
The commander ruled out any negotiations while foreign troops were in the country and threatened dire consequences for anyone who held talks.
"Those who negotiate in the name of the Taliban will be killed," he said.
The government has a reconciliation programme aimed at persuading Taliban members to give up their fight and rejoin societ,y but few have taken up the offer.
Some Afghan politicians have said that peace will be impossible unless elements of the Taliban are included in talks.