New signs emerged Tuesday of US frustration with Afghanistan's President Hamid Karzai, with a report that Washington may quicken its troop withdrawal or even leave no forces behind after 2014.

The New York Times reported that both options were being seriously considered following a tense teleconference between Obama and Karzai late last month.

It was unclear however, whether the administration was seeking to pressure Karzai following a spat with the White House over peace talks with the Taliban.

Pentagon spokesman George Little said Obama "is still reviewing options from his national security team and has not made a decision yet about the size of the possible US presence after 2014."

General Joseph Dunford, the US commander in Afghanistan, "has noted that we have time and space to make a decision on troops levels beyond 2014," he said.

"Any potential US military presence beyond 2014 would focus on two basic missions: targeting the remnants of Al-Qaeda and its affiliates and training and equipping Afghan forces," Little added.

The idea of a "zero option" of leaving no troops behind was first mooted earlier this year by US deputy national security advisor Ben Rhodes.

And US administration officials stressed Tuesday that the idea had been around for a while and was not new.

Obama was still reviewing the options and no decision had yet been made, insisted State Department spokeswoman Jen Psaki.

The zero option "has been an option that has been on the table for quite some time," she said.

But she stressed: "We have been clear in public and in private, as have many of our allies and partners in ISAF and in the broader international community, that we do not intend to repeat the mistakes of the '80s and '90s and that as the Afghans stand up, they won't stand alone."

But she refused to elaborate on what mistakes had been made and by whom.

White House spokesman Jan Carney meanwhile pointed to US-Afghan bilateral security talks, saying "these are ongoing conversations."

Obama is committed to bringing US military involvement in Afghanistan to an end by 2014, as part of a core project of ending foreign wars.

But his administration is negotiating with Karzai on leaving behind a "residual" force to fight any renewed terror threat and to train Afghan forces.

The US relationship with Karzai, while never good and often volatile, deteriorated again last month over stillborn peace talks with the Taliban in Doha.

Kabul said Sunday that the talks on a post-2014 US presence in Afghanistan could only resume once Taliban representatives meet with Karzai's peace negotiators.

Karzai was infuriated when the Taliban opened an office in the Doha, the Qatari capital seen as a venue for the talks, then portrayed it as an embassy for an alternative Afghan government.

To defuse tensions, Obama and Karzai spoke by videoconference June 27, but the session went badly, the Times said, quoting American and Afghan officials familiar with the exchange.

They said Karzai accused the United States of trying to negotiate a separate peace with both the Taliban and their backers in Pakistan. Karzai felt this would leave his country exposed to hostile neighbors.

US officials said the United States was still trying to reach an agreement with Kabul on the future NATO security presence.

But negotiating stances are hardening, said the Times, citing senior US and European officials.

"There has always been a zero option, but it was not seen as the main option," the Times quoted a senior Western official in Kabul as saying.

"It is now becoming one of them, and if you listen to some people in Washington, it is maybe now being seen as a realistic path."

Many in the administration may see a political upside to leaving no American troops in harm's way in Afghanistan after the longest US war comes to a close.

But some security experts question whether Afghanistan's nascent armed forces can stand up to the Taliban on their own.

They also warn that US efforts to eliminate Al-Qaeda remnants in volatile Pakistani tribal areas could be hampered without NATO forces on the Afghan side of the border.

Currently, half of the 68,000 US troops in Afghanistan are set to exit by February, and the newly-trained Afghan army and police are increasingly taking the lead on the battlefield.

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