The Taliban and Pakistan's spy agency denied Thursday a report suggesting that Mullah Omar had suffered a heart attack and was treated with the help of the intelligence network.

Both poured scorn on a Washington Post report on Tuesday that quoted a private intelligence network saying the Taliban's spiritual leader had been hospitalised in Pakistan's Karachi city and had suffered some brain damage.

The Post said the one-eyed Islamist, who disappeared after the 2001 US-led invasion of Afghanistan, appeared to have suffered a heart attack on January 7 and had been taken to a hospital near the southern metropolis.

Taliban spokesman Zabihullah Mujahib told AFP: "Supreme leader Mullah Mohammad Omar is absolutely fine and healthy and is continuing his jihad activities in Afghanistan.

"We were informed of this news through media only and his heart surgery in Karachi is a mere rumour created by our enemies," he added.

The newspaper cited a report by a private intelligence network run by former US security officials, known as "The Eclipse Group," which reportedly said its source was a physician at the hospital.

The Post said Inter-Services Intelligence (ISI) agency had "rushed him to a hospital in Karachi, where he was given heparin (an anti-coagulant) and operated on," and that he was released to the ISI after a few days.

As well as a heart attack, the doctor reportedly said Omar appeared to have suffered some brain damage and slurred speech following an operation conducted after the heart attack.

A spokesman for Pakistan's military, of which the ISI is a part, "strongly contradicted the news item... about the medical treatment of Afghan Taliban Mulla Mohammad Omar at Karachi," in a statement.

"(The) news is unfounded and concocted to serve vested interests," the statement added.

Although Pakistan is officially an ally of the international coalition battling the Taliban in Afghanistan, US officials have long accused the ISI of playing a double game.

The ISI is reportedly still close to Omar, whom it backed as the leader of the 1996-2001 Taliban regime in Kabul.

Pakistan has always strongly denied such charges, and the Post quoted its Washington ambassador Husain Haqqani as saying that the Eclipse report "had no basis whatsoever."

The Taliban's Mujahib accused countries with troops in Afghanistan of spreading the rumours over Omar's health.

"The enemy is trying to cover their losses and create tension and terror among the people of Afghanistan and want to show that the Afghan government and their Western allies are the winners," he added.

The Taliban say publicly that international forces must leave Afghanistan before they will consider peace talks after nine years of war.

Rumours of peace talks persist, however, with the Post reporting in October that Mullah Omar and his Quetta Shura, the Afghan Taliban group based in Pakistan, have held secret negotiations.

There are around 140,000 international troops in Afghanistan, and the coalition is due to start a limited, conditions-based withdrawal from July, with Afghan forces scheduled to take over responsibility for security in 2014.