U.S. soldiers investigated a Sept. 8 suicide bombing south of Kabul. U.S. officials are debating the size of a potential U.S. force beyond year's end. Agence France-Presse/Getty Images
WASHINGTON—Vice President Joe Biden has resumed a push to withdraw virtually all U.S. troops from Afghanistan at year's end, arguing for a far-smaller presence than many military officers would like to see, said officials briefed on the discussions.
The White House convened a meeting of top national-security officials on Thursday to discuss the war and the future of the U.S. troop presence.
Mr. Biden has lost previous debates on Afghanistan, but his arguments for a smaller force, likely of 2,000 to 3,000 troops, have gained traction within an administration increasingly frustrated by Afghan President Hamid Karzai. Mr. Karzai has refused to sign a security agreement allowing American forces to remain in small numbers after the North Atlantic Treaty Organization mission there formally ends this year.
Some U.S. defense officials, preferring a remaining post-2014 U.S. force of 9,000-12,000, are skeptical of the smaller troop presence Mr. Biden and others advocate. Such a force would be so limited that a full pullout would make more military sense, the officials said.
"We are coming to grips with the potential for zero," said a military official.
A senior administration official said Mr. Biden hasn't advocated for any specific number of forces.
"He has not rejected any specific troop level," the official said. "He has asked questions and listened carefully to presentations and he will make his recommendation at the appropriate time."
The resumption of the administration debate and the push by Mr. Biden and his allies in the administration for a limited force concerns members of groups who advocate for continued U.S. engagement. They fear a debate focused on a small force would offer little appeal to the Afghan government, prompting Mr. Karzai to refuse to sign the security agreement and the Obama administration to withdraw all U.S. forces.
"Pulling the rug out from under Afghanistan really risks collapse," said Andrew Wilder, vice president of South and Central Asia programs at the United States Institute of Peace. "We're in the endgame with Karzai, hopefully, and we really risk blowing it by announcing a 'zero option' based on our frustrations with negotiating with a president who should soon be gone."
The current White House discussions, said people familiar with them, are an echo of the debate over Afghanistan strategy in 2009, when Mr. Biden argued for a smaller force focused on counterterrorism operations.
This time, however, the disagreements within the administration largely have been kept behind the scenes. Even the most vocal supporters of a long-term troop presence, including top military officers, have toned down their public protests about the prospects of a complete withdrawal.
Officials said Defense Secretary Chuck Hagel is supportive of the recommendations of his military leaders, including the belief that a force smaller than 9,000 would be ineffective.
While Mr. Hagel has been skeptical of long-term overseas deployments, many of his military advisers support the higher troop level. U.S. military officials have outlined to administration officials the risk of the kind of small deployment backed by Mr. Biden, especially if the force were divided among three bases.
However, military leaders don't want another public clash with the White House, officials said. "The military is not pushing hard on this," said the military official. "If the decision is zero, we will be supportive. We believe in the mission, but we've got plenty of other missions as well."
Former Defense Secretary Robert Gates criticized Mr. Biden's position in the 2009 Afghanistan debate in his memoir released this week, saying the vice president was wrong to argue against a larger troop presence. But Mr. Gates also took issue with military leaders who publicly advocated for more troops ahead of Mr. Obama's decision.
In the current discussion, Gen. Joe Dunford, the top commander in Afghanistan, and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, have privately offered their advice to the White House. But both have been careful not to be seen as publicly backing higher troop numbers.
"Gen. Dempsey is a student of history, from [Gen. Douglas] MacArthur and [President Harry] Truman to more recent history," said a military officer. "He does not want to limit the president's decision space."
The recent discussions about Afghanistan are taking place amid deteriorating security in western Iraq, which some policy makers and military officials blame on a vacuum resulting from the total U.S. withdrawal from that country in 2011.
United Nations Ambassador Samantha Power and other key U.S. officials have made it clear in recent meetings that the zero option is becoming increasingly likely because of Mr. Karzai's refusal to sign the deal, said people involved in discussions about the administration's Afghan policy.
Administration officials said planning to withdraw all international forces could become more pressing after a Feb. 22 NATO meeting if no deal with Mr. Karzai on the security agreement is worked out by then. Administration and military officials say a unified message is critical to force Mr. Karzai's hand.
"The Afghans need to understand that not signing the BSA makes the zero option a reality," said a senior State Department official.
Mr. Karzai has brushed off threats from the U.S. administration and said he won't sign the agreement until American officials make some headway in peace talks with the Taliban. He has also has expressed distrust of the U.S. and a belief that the Obama administration is bluffing when it threatens to leave Afghanistan entirely, said people who have met the Afghan president in recent weeks.
—Adam Entous and Jay Solomon contributed to this article.
Write to Dion Nissenbaum at dion.nissenbaum@wsj.com, Julian E. Barnes atjulian.barnes@wsj.com and Carol E. Lee at carol.lee@wsj.com