US Defence Secretary Robert Gates is expected to unify the military command for US troops in Afghanistan and has endorsed a multi-billion-dollar plan to nearly double the size of the Afghan army, officials have said.

The changes are part of the Pentagon's latest response to a growing challenge from the Taliban insurgency, particularly in eastern Afghanistan, where officials say US and Nato forces are battling militants trained at safe havens in neighbouring Pakistan.

US officials are looking at ways to send more US troops to Afghanistan, including the possible redeployment of combat brigades now earmarked for Iraq.

But so far, the Pentagon has opted to enhance the force already on the ground by extending tours for more than 3,000 Marines while deploying about 175 support troops including helicopter units to boost the Marines' fighting capacity.

In another action intended to make US forces more effective, Gates is expected soon to sign an order putting the Nato commander in Afghanistan, US Army General David McKiernan, in charge of nearly all US forces that are not connected with Nato's 53,000-strong International Security Assistance Force.

The United States has 34,000 troops in Afghanistan, including about 15,000 already under McKiernan's command as part of ISAF. The anticipated change would give him most of the remaining 19,000 troops currently under the separate US command known as Operation Enduring Freedom.

The Nato and OEF missions would remain separate, but the switch would minimise confusion within the US command ahead of expected troop deployments, possibly next year, officials said.

"The number of our troops in Afghanistan is only going up, and so we want to make sure that as we're growing, that we have the most efficient and effective command structure in place," said Geoff Morrell, Gates' press secretary.

Gates is discussing the change with his Nato counterparts and could order the command change before August 31, according to an official who spoke on condition of anonymity.