In a statement read by Mohammad Hanif, a Taliban spokesman, over the telephone from an undisclosed location, Mullah Mohammad Omar said: "With the arrival of the warm weather, we will make the ground so hot for the invaders it will be unimaginable for them."
An insurgency that has killed more than 1500 people since the start of last year has already intensified in recent months.
Ten US soldiers have been killed in combat this year and US commanders have said they expect fighting to increase as the weather warms and snow on mountain passes melts.
The Pakistan-based Afghan Islamic Press news agency reported Omar as saying many young Afghans were volunteering for missions.
Last week, Hamid Karzai, the president of Afghanistan, called for more Pakistani co-operation after Islamabad derided Kabul's accusation that Omar was in Pakistan.
On Wednesday, Abdullah Abdullah, Afghanistan's foreign minister, said he was sure that the Taliban leader was not in Afghanistan, although Taliban spokesmen insist that Omar is leading the insurgency from his homeland.
"Mullah Omar is not in Afghanistan, that's as much as I can say with a degree of certainty," Abdullah said during a visit to the Malaysian capital, Kuala Lumpur.
Afghan officials complain that the Taliban uses Pakistan's tribal regions as a springboard for attacks, and despite Islamabad's denials, many suspect that Pakistan harbours long-term ambitions to have a pro-Pakistan government in Kabul.
A US commander said last week an increase in violence was expected as US and Nato forces extend their reach into parts of Afghanistan where the insurgent presence is greater.
Rear-Admiral Robert Moeller, the US Central Command director, told a congressional hearing: "We anticipate that we are going to see a fairly violent spring and summer and then an improvement in overall conditions."
"We anticipate that we are going to see a fairly violent spring and summer and then an improvement in overall conditions"
Rear Admiral Robert Moeller,
US Central Command director
The 26-member Nato alliance is preparing to expand its International Security Assistance Force mission - already in the north and west of the country and in the capital, Kabul - to the more volatile south and ultimately the east, raising its troop numbers to 16,000 from 9000.
About 18,000 US soldiers in the country are fighting Taliban and al-Qaida forces, but the US hopes to cut numbers by several thousand as Nato forces take on more responsibilities and the Afghan army becomes stronger.
Pakistan has deployed about 80,000 soldiers in frontier areas to try to stop fighters from moving across the border, and it co-ordinates with US and Afghan forces on the other side.
The Taliban took power in Kabul in the mid-1990s with Pakistan's backing.
Under US pressure, Pakistan abandoned support for the Taliban in late 2001, after its leaders refused to surrender Osama bin Laden, the leader of al-Qaida, after the 11 September 2001 attacks on New York and Washington.