The border buildup was to be announced today - two days before civilian volunteers with the so-called Minuteman Project begin a monthlong Arizona patrol against immigrants crossing the U.S.-Mexico line.
About 155 agents will be sent to Arizona immediately, according to department officials who spoke on condition of anonymity because the buildup was not yet announced. More than 370 additional agents - all new trainees - will be permanently assigned to the Arizona border throughout the year.
Until they are in place, another 200 agents will be temporarily stationed in Arizona during the high immigration season this spring and summer, officials said.
A Senate aide confirmed Tuesday that more agents would be assigned to Arizona, but couldn't say exactly how many were involved.
The 370-mile Arizona border is considered the most vulnerable stretch of the 2,000-mile southern border.
Of the 1.1 million illegal immigrants apprehended by the U.S. Border Patrol last year, 51 percent crossed into the country at the Arizona border.
Recent intelligence indicates that al-Qaida leaders are likely to enter the country through the Mexico border and ''believe illegal entry is more advantageous than legal entry for operational security reasons,'' former Homeland Security Deputy Secretary James Loy said in written testimony to lawmakers last month.
The new agents will come on top of the 210 that President Bush has proposed for all U.S. borders in his budget last month - a number lawmakers have called inadequate to effectively secure the nation's borders. It also falls far short of the 2,000 new agents mandated in intelligence reform legislation enacted in December.
Hundreds of civilian ''Minuteman'' volunteers have signed up to patrol a 40-mile stretch of the southeast Arizona border. They say they will merely identify and follow illegal border-crossers and not interact with them. But some of the volunteers plan to arm themselves, although they have little or no training to confront border-crossers.
T.J. Bonner, president of the National Border Patrol Council union, questioned whether the new employees would be enough.
''Right now, things are so out of control, we have no idea who's crossing our borders, and we can't but chase after but a few of the people,'' said Bonner, whose union represents more than 9,000 agents. ''It's going to take more than a couple of hundred agents to seal those gaps.''
An estimated 2,400 agents currently patrol the Arizona border - about a quarter of the 10,000 assigned to the southern border. The 155 newly assigned agents will be pulled from elsewhere along the southern border - and not from the northern border with Canada, Homeland Security officials said.
Of the 200 temporarily assigned agents, 26 are specifically trained for search and rescue operations, officials said. Last year, 330 migrants died - mostly from exposure to the elements and lack of food and water - while crossing the southwest border. Officials said Border Patrol agents rescue between 1,200 and 1,500 migrants in the southwest annually.