The Taliban in Afghanistan remain utterly opposed to peace talks despite slow progress toward reconciliation, their leader said on Monday after NATO forces suffered their worst losses in months in a spike in violence.

Mullah Mohammad Omar, the secretive, one-eyed leader of the Afghan Taliban, issued a statement just four days before NATO leaders will gather for a summit in Lisbon where Afghanistan will be the top of the agenda.

Violence across Afghanistan was already at its worst since the Taliban were ousted by U.S.-backed Afghan forces in late 2001 but a dramatic increase in attacks in the past four days will be a sobering message for NATO leaders.

Military and civilian casualties are at record levels and while both sides have been talking up recent success on the battlefield, there is growing acceptance of the need for a negotiated settlement to the conflict.

Omar reiterated that would not be possible until all foreign troops -- now at about 150,000 -- left Afghanistan, labeling talk of negotiations "mere propaganda."

"The cunning enemy which has occupied our country is trying, on the one hand, to expand its military operations ... and, on the other hand, wants to throw dust in the eyes of the people by spreading the rumors of negotiation," Omar said in a statement.

Afghan President Hamid Karzai has included eventual talks with the Taliban as part of a wider reconciliation plan but Afghan and U.S. officials have played down confused and unconfirmed reports about talks with high-level insurgents, saying any contacts so far have only been preliminary.

Karzai's peace plan also will be addressed in Lisbon, with Afghanistan setting the ambitious target of 2014 for its forces to take over complete security responsibility.

U.S. President Barack Obama plans to begin a gradual drawdown of U.S. forces from July 2011. The chance of talks and the recent surge in violence also will play on his mind when he reviews his Afghanistan war strategy next month.

Many European NATO leaders are under pressure from an increasingly sceptical public to justify their continued support for the costly and drawn-out conflict.

In a weekend interview with The Washington Post, Karzai left little doubt that he thought it was time for foreign troops to begin cutting back on operations and reducing their visibility.

The interview also underscored his often uneasy relationship with Washington, which Karzai described as "grudging," although his office tried to smooth over the appearance of division.