The Pentagon, which makes heavy use of private security contractors in Afghanistan, said Afghan President Hamid Karzai's four-month deadline for eliminating them was "very aggressive."

Karzai signed a decree on Tuesday ordering the dissolution of all the security firms by January 1, a move certain to create major problems for the government's Western backers at a time of mounting insurgent violence.

"With respect to a timeline of four months, obviously, that's a very aggressive timeline," said Pentagon spokesman Bryan Whitman.

The private security firms, which employ up to 40,000 people, many of them Afghans, are used to protect everything from convoys to bases, embassies and non-governmental relief operations.

But they have also stirred resentment among ordinary Afghans for heavy-handed tactics on the country's roads.

The Karzai government also argues that their use is diverting resources from the training and equipping of Afghan security forces.

Whitman said the Pentagon shared Karzai's goal of eliminating the private security firms or bringing them under the control of the central government.

"We also recognize that Afghanistan still represents a daunting security challenge," he said.

He said the Pentagon wanted to work with the Afghan government "to achieve this goal in a deliberate way that recognizes both the scope and the scale of the challenge that's presented by this."