WASHINGTON:The Pentagon said Tuesday that ''substantial'' long-term military support will be needed to ensure that Afghans can hold off the Taliban insurgency after the US combat mission ends in December 2014.

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 Defense Department said Afghanistan's military is growing stronger but will require a lot more training, advising and foreign financial aid after the US and Nato combat mission ends.

The Obama administration has pledged to stand with the Afghans for the long term. But President Barack 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.

They take into account the Afghans' need for additional training and advising, plus what the Pentagon sees as a longer-term requirement for US counterterrorism forces in Afghanistan.

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

The US has not yet successfully negotiated a security deal with Kabul that gives the US a legal basis for having forces in Afghanistan after 2014.

Talks began last year but have made little recent progress.

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 has the support of the United States and other countries, but it infuriated Karzai after the Taliban opened it with a display of the name and flag it used when it ruled Afghanistan.

Relations between Afghanistan and the United States plunged after that incident. There are currently about 60,000 US troops in Afghanistan, down from a 2010 peak of 100,000, and the total is to shrink to 34,000 by February.

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.

The report said 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 US and other coalition partners has declined. But 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.''