WASHINGTON (AP) — The Pentagon said Tuesday it is offering no "zero option" for the number of troops that would remain in Afghanistan after the U.S. combat mission ends in December 2014. It said in a report to Congress that "substantial" long-term military support will be needed to ensure that Afghans can hold off the Taliban insurgency.

The White House has not ruled out leaving no troops behind after 2014, although officials say the most likely option is a residual training force of roughly 9,000.

In its twice-a-year report to Congress on war progress, the Pentagon said Afghanistan's military is growing stronger but will require a lot more training, advising and foreign financial aid after the American and NATO combat mission ends.

The Pentagon's assessment was an implicit rejection of the "zero option." Zero is considered an unlikely choice by President Barack Obama, not least because his administration has pledged to stand with the Afghans for the long term. But Obama has grown frustrated in his dealings with Afghan President Hamid Karzai.

Peter Lavoy, the Pentagon's top Afghan policy official, told a news conference that a number of post-2014 options have been developed, taking into account the Afghans' need for additional training and advising, as well as what the Pentagon views as a longer-term requirement for U.S. counterterrorism forces in Afghanistan.

"In none of these cases have we developed an option that is zero," Lavoy said.

It remains possible that the administration will be left with no option other than zero if it cannot successfully negotiate a security deal with Kabul that gives the U.S. a legal basis for having forces in Afghanistan after 2014.

Talks on a security deal began last year but have made little recent headway. Karzai suspended negotiations following a disagreement this month over a Taliban political office in the Gulf state of Qatar that was to host peace talks. The office, which has the support of the United States and other countries, infuriated Karzai after the Taliban opened it with a display of the name and flag it used when it ruled Afghanistan.

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Relations between Afghanistan and the United States plunged after that incident.

In its report to Congress, which is required by law every six months, the Pentagon made no recommendation on the number of U.S. troops to keep in Afghanistan after 2014. There are currently about 60,000 U.S. troops there — down from a 2010 peak of 100,000 — and the total is to shrink to 34,000 by February.

The report said it will be difficult to judge whether Afghanistan can keep the upper hand against the Taliban until the exact size of a post-2014 U.S. military presence is determined.

The report painted a largely positive picture of progress in strengthening the Afghan army and police, but it offered cautionary assessments of the economic and political elements of its strategy for stabilizing the country.

"Effective government, the rule of law and sustainable economic development are all necessary for long-term stability in Afghanistan, but multiple factors continue to hinder them, including widespread corruption," it said.

The report said that the amount of Afghan territory held by the insurgents has continued to shrink. It called the Taliban "less capable, less popular and less of an existential threat" to the Kabul government. And it said the number of "insider attacks" by Afghan forces against their U.S. and other coalition partners has declined.

On the other hand, it said the insurgents still wield influence in several key rural areas that serve as avenues to attack urban areas, including certain districts adjacent to Kabul and in areas west of the southern city of Kandahar.

"Insurgents also used violence and assassination to undermine perceptions of the Afghan government's ability to provide security," the report said, "including intimidation of tribal elders, local power brokers and Afghan government officials."

For the first time, the report to Congress said some Afghan security forces are making potentially troublesome accommodations — in some cases in the form of local ceasefire deals — with insurgent groups.

Lavoy said these are generally desirable moves toward reconciliation with the Taliban, but the report said they "can have negative effects" if Afghan military leaders compromise on security standards in the process. It said these deals have the potential to "throw off the balance of power in a given area, causing more destabilization if agreements are achieved under coercion or co-option." More deal-making is likely as U.S. and other foreign forces leave the battlefield, the report said.

The report also provided the first detailed public explanation of a mix-up earlier this year in which the U.S. military was forced to retreat from a claim that Taliban attacks had declined by 7 percent last year, compared to 2011. After inquiries by The Associated Press, the military determined that there had been no decline at all.

Tuesday's report said the incorrect report of a 7 percent decline did not take account of a number of attacks that Afghan government forces had accurately reported but that the U.S.-led military command in Kabul had failed to record. It said the error, discovered in January, did "slightly affect" the data that was provided in the Pentagon's previous report to Congress, published last December, but did not change broad trends.

The report said statistics on attacks, which had previously been a central feature of the Pentagon's reports to Congress and in other U.S. government assessments of war progress, will no longer be used in that way. One reason is that the data now come mainly from Afghan rather than coalition forces, since the Afghans are now doing most of the fighting; as a result, the figures are less detailed and take longer to process.

"It is now clear that (coalition) public reporting and media coverage of the war have relied too heavily on (attack) reporting as a 'scoreboard' for progress," the report said.

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