US troops will stay in Afghanistan after 2014 "only at the invitation" of the Afghan government, and Washington is not seeking permanent bases there, a White House spokesman said Thursday.

White House press secretary Jay Carney told reporters traveling with US President Barack Obama to Texas that any US troop presence after that date would be "only at the invitation of the Afghans" and "subject to an agreement."

Afghan President Hamid Karzai said earlier he could allow the United States to keep nine military bases in the country as part of a long-term security pact with Washington.

"As the president has already made clear, the United States does not seek permanent military bases in Afghanistan, and any US presence after 2014 would only be at the invitation of the Afghan government and aimed at training Afghan forces and targeting the remnants of Al-Qaeda," Carney said.

He said the bilateral security agreement that is currently being negotiated would deal with the bases issue.

The back-and-forth comes as the two countries are negotiating a deal to allow a limited US troop presence to remain when the international coalition leaves next year.

The size of the "residual" force has not been agreed, with numbers ranging from 2,500 to 12,000, according to US officials, as Washington winds down an 11-year-old war that has become deeply unpopular at home.