Retired General George Joulwan said the use of guardsmen in a supporting role could be justified, given the seriousness of the border problem, but pointed out that the Guard was already overstretched.

However, John Goheen, spokesman for the National Guard Association, an advocacy group for Guard officers, said with about 444,000 personnel across the United States, "it would seem this mission is doable".

"The numbers would not seem to make it that difficult."

At one point last year National Guard troops represented about 50% of all US combat forces in Iraq, but that has dropped sharply in recent months.

There are now about 17,000 members in Iraq and that number is not expected to rise significantly this year or next.

Hurricanes Katrina and Rita also put an extra burden on the Guard last year, requiring the use of 50,000 troops. 

National Guard troops already provide support to law enforcement agencies on the border, although in small numbers. 

Supporting role

Those selected for duty on the border with Mexico will not chase down illegal immigrants but will play behind-the-scenes roles in support of border guards, officials said.

Among the tasks they are likely to perform: Training federal border patrol guards, who are civilians; building barriers near the border; improving roads; providing support for aerial and ground surveillance; analysing and sharing intelligence; providing communications systems and transportation.

It was not clear whether the troops would come only from the four border states - California, Arizona, New Mexico and Texas.

The National Guard is a military force under the command of state governors unless federalised by the president. Tens of thousands of guardsmen have been deployed with regular fighting forces in Iraq and Afghanistan.

Defence officials stressed that the intention of sending troops to the border was to provide short-term assistance until the Border Patrol and other agencies can develop more of their own capabilities.