The move - involving the seldom-tapped Individual Ready Reserve - represents the latest evidence of the strain being placed on the US military, particularly the army, by operations in those two countries.
Roughly 5600 soldiers from the ready reserve will be notified of possible deployment this year, including some soldiers who will be notified within a month, said an army official speaking on condition of anonymity.
A senior defence official said: "These individuals are being called back to fill specific shortages for specific jobs."
The official said the last time the Individual Ready Reserve, mainly made up of soldiers who have completed their active duty obligations, was mobilised in any significant numbers was during the 1991 Gulf war.
Army officials are in the process of briefing members of Congress on the mobilisation and plan a formal announcement on Wednesday.
The army official said the mobilisation "will be through the rest of the year. Some could be within a month".
"It would be an involuntary measure, an involuntary mobilisation," the official said. "It's approximately 5600."
"We're not calling up units, we're just using all the existing assets in theatre and we're augmenting those assets with these individuals - various occupational specialties,
Currently 20,000 US troops are
deployed in Afghanistan alone
various different types of officers running the whole gamut," the official stressed.
The official added that military police and civil affairs personnel were among the specialties involved.
The defence official said that while soldiers in the Individual Ready Reserve have served their voluntary obligation in the army, they still can be mobilised involuntarily for several years after returning to civilian life.
"Sometimes there's a misperception by some of the individuals ... that 'I've done my obligation, I've been in the army, thank you very much, and I'm done'. But you're not done," the official said.
The Pentagon had originally planned to reduce the number of US troops in Iraq to approximately 110,000 by now, but continuing security problems compelled officials to maintain a level of about 138,000 troops. Officials have said they planned to maintain that number through to the end of 2005.
The United Sates has another 20,000 troops in Afghanistan.
This spring, the Pentagon delayed by about three months the scheduled departure from Iraq of roughly 20,000 US troops.