WASHINGTON - The so-called "zero option" - the complete withdrawal of all U.S. troops from Afghanistan by year's end - could result in a civil war and a mass desertion of Afghan security forces, a senior Pakistani official warned today.

The White House acknowledged for the first time today that it is unlikely that Afghan President Hamid Karzai will sign a security agreement that would allow a small force of American troops to remain in Afghanistan. As a result, it said the Pentagon had been tasked with drawing up plans that could result in the full withdrawal of American troops from Afghanistan should an agreement not be signed.

A senior Pakistani official speculated the Obama administration's announcement is "posturing" intended to get Karzai to sign the agreement. However, he also said American policy makers should be wary of the potential consequences a pullout could have on Afghanistan and the region.

"In my opinion the zero option should not be an option," said the senior Pakistani official. "Zero option means a civil war in Afghanistan."

The official considers Afghanistan's security forces to be so weak that he fears a full U.S. troop withdrawal could lead 30 percent of Afghanistan's 352,000 security forces to desert.

"Leaving them on their own will not be productive," the official said. "This is a very dangerous thing."

The Pakistani official spoke with reporters on the condition of anonymity at a gathering organized in Washington by the Center for Media and Security.

The senior Pakistani official indicated that a full withdrawal of U.S. troops could also have "drastic effects" inside Pakistan as it could lead to a spillover of terrorist activity. Pakistan also fears that renewed violence inside Afghanistan could trigger a new refugee crisis, with the official estimating that as many as 2 million refugees could flood into Pakistan, which is already home to 2.5 million Afghan refugees.

Using dire language to describe both scenarios, the official said "it will be holocaust, it will be terrible."

U.S. officials say the White House is considering a Pentagon proposal that the U.S. keep a force of 10,000 in Afghanistan for two additional years beyond 2014 to continue to train and support the Afghan military.

The White House is also considering another proposal that would keep as few as 3,000 forces focused on a counterterrorism mission. A decision on a final number of troops has been delayed while the Obama administration waited for Karzai to sign an agreement he negotiated late last year.

The senior Pakistani official said the size of any American force that remains in Afghanistan is not as important as the continued logistical and helicopter lift they can provide to Afghanistan's military.

There are currently 33,600 U.S. troops serving in Afghanistan, 13 years after the Sept. 11 terror attacks. The official said it would be a "tragedy" for the United States and its NATO allies to "walk out" on Afghanistan after such a long stay after "so many lives lost."

The official said Pakistan intends to keep the pressure on the Taliban by maintaining its current force levels of 150,000 troops in the tribal areas of western Pakistan for the next one to three years.

The Pakistani government has recently engaged in peace talks with the Pakistani Taliban in North Waziristan, though those talks have now stalled.

The official acknowledged that though a negotiated deal is the preferred option, Pakistan's military is prepared to launch a significant military action against Taliban militants should the talks fail.

"There will be operations not only in North Waziristan but wherever necessary," said the official, who also predicted that the violent Haqqani Network "will be reaching its conclusion once North Waziristan is cleared."

The official acknowledged that in spite of a moratorium on military activity during the talks, Pakistan's military has conducted "retaliatory strikes" in the wake of recent violent Taliban attacks, including the execution of 23 Pakistani soldiers and a bomb attack in Karachi that killed 19.