US leaders will discuss whether to prolong the timetable for the withdrawal of American troops with Afghan President Ashraf Ghani when he visits Washington early next year, a senior US official said.

President Barack Obama's administration was prepared to discuss with Ghani "whether adjustments need to be made to the planning that was originally forecast for our presence there," Peter McKinley, Obama's pick to be the next US envoy to Kabul, told US senators.

Obama has invited the new Afghan president to visit the US early next year, said McKinley, who is the current deputy US ambassador to Kabul. The exact date has not yet been confirmed.

"In addition to the panoply of issues related to the bilateral relationship, we do anticipate security issues and the future of our relationship with Afghanistan to be part of that discussion," McKinley told his nomination Tuesday.

The post-2014 NATO mission calls for 12,500 troops to remain on the ground starting in January, including about 2,700 non-US forces.

But delays in signing a bilateral security deal and a protracted presidential election have raised doubts about whether allied governments can have arrangements in place for troops to deploy by 2015.

Defense officials told AFP last week the US military likely will keep a slightly larger force in Afghanistan in 2015 than first planned and American troops will have authority to come to the aid of Afghan forces if necessary.

The United States may also have to deploy hundreds of additional forces in coming months, beyond the 9,800-strong contingent announced previously, to meet an estimated 400 to 700 troops shortfall.

Experts have also questioned the wisdom of pulling out all US troops by the end of 2016, amid a strengthening insurgency by Islamic Taliban militants.

"The facts on the ground are that militarily the Taliban is still very strong and capable of mounting serious attacks, including inside the capital," said veteran Republican senator John McCain.

"And the message that has been sent is we're going to be out."

He called on the Obama administration to ensure that "our withdrawal will be conditions-based rather than dictated by a certain calendar."

McKinley pointed to a national security review being undertaken by the new Afghan government to "find a more effective way to respond to the challenge" posed by the Taliban.

Washington would "consult closely with Afghanistan's leaders on security issues, to include a discussion of possible refinements to our plans regarding the mission's duration," he added.