The US military was looking at freeing up more troops for combat in Afghanistan as President Barack Obama warned there would be no "quick" victory in the war.

With Afghans heading into crucial elections Thursday, US defense officials said the commander of US and NATO forces in Afghanistan, General Stanley McChrystal, was weighing cutting back desk jobs and other support staff to free up more soldiers for combat.

"The idea is use troops more effectively," a US official, who spoke on condition of anonymity, told AFP on Monday.

Reducing non-combat positions would mean "doing more with what you''ve got versus asking for more" troops, the official said.

McChrystal is taking a hard look at the war against Taliban insurgents amid widespread speculation he may soon ask Obama for additional US forces -- a politically-charged issue at home and abroad.

Cutting the number of support staff could mean the US general would make a more modest troop request, possibly easing pressure on Obama who faces rising anxiety over the war within his own party.

As candidates held rallies at the close of campaigning in Afghanistan, Obama defended the war as a necessary mission but warned of a difficult road ahead.

"The insurgency in Afghanistan didn''t just happen overnight," Obama told the Veterans of Foreign Wars service organization in the southwestern state of Arizona.

"We won''t defeat it overnight. This will not be quick. This will not be easy," Obama said.