Taliban insurgents have rejected a United States offer of "honourable reconciliation" as a "lunatic idea" and said the withdrawal of foreign troops was the only way to end the war in Afghanistan.

With the Afghan conflict now in its eighth year, NATO-led forces and the Taliban are locked in a bloody stalemate with violence set to rise further this year as more US troops arrive and seek to contain the insurgency ahead of August elections.

President Barack Obama is redoubling US efforts with more troops, more diplomatic effort and more economic assistance, but he has also already spoken of the need for an "exit strategy".

If the US plan fails to show results, analysts say, time is on the Taliban side.

US Secretary of State Hillary Clinton told an international conference on Afghanistan on Tuesday that those members of the Taliban who abandoned extremism must be granted an "honourable form of reconciliation".

"This matter was also raised in the past," said Taliban spokesman Zabihullah Mujahid, referring to comments by Mr Obama last month who spoke of reaching out to moderate Taliban.

"They have to go and find the moderate Taliban, their leader and speak to them. This is a lunatic idea," Mr Mujahid said by telephone from an unknown location.

The 21,000 extra US troops ordered by Mr Obama to join the 70,000 foreign soldiers now fighting insurgents in Afghanistan showed the United States wanted the war to continue, Mr Mujahid said, and the Taliban would keep fighting till they left.

"There is no other way. We want our freedom and respect for our independence," he said.

- Reuters