"We do not yet have full control of the border, and I am determined to change that," George Bush said in a prime-time Oval Office address timed to get out in front of a US Senate debate on a sweeping immigration overhaul.
Bush also called on Congress to provide $1.9 billion emergency funding for dramatic improvements in manpower and technology at the border.
"The need to secure our border is urgent," he said.
White House officials described the National Guard deployment as temporary and said the troops would not be involved in law enforcement.
Some members of Congress feared the National Guard, already with 17,000 members in Iraq, will be stretched too thin, and Mexico worried about militarising the border with soldiers.
"It is important for Americans to know that we have enough Guard forces to win the 'war on terror', respond to natural disasters, and help secure our border," Bush said.
He also said that "the United States is not going to militarise the southern border", adding: "Mexico is our neighbour and our friend."
White House officials said the troops would provide logistic support for US Border Patrol agents whose job is to arrest illegal immigrants crossing into the states of Arizona, California, New Mexico and Texas.
Up to 6,000 National Guard
troops would be used for a year
The deployments are likely to begin in early June and up to 6,000 would be used for a year. They will be reduced as US Border Patrol agents increase their number by 6,000 by the end of 2008 - from 12,000 to 18,000.
Analyst believe the president was responding to mounting pressure from conservative allies to take tougher steps against illegal immigration.
But Bush also insisted on a temporary guest-worker programme for illegal immigrants that would let them fill jobs Americans refuse to do.
Bush said border enforcement alone would not solve the problem because many people would do anything to get to the US.
"This creates enormous pressure on our border that walls and patrols alone will not stop," he said. "To secure the border effectively we must reduce the numbers of people trying to sneak across."
Bush was hoping to influence a Senate debate this week on an immigration overhaul that would couple tougher border enforcement with a temporary guest-worker plan and create a mechanism for many of the estimated 12 million illegal immigrants in the country to legalise their status.
And he was hoping to sway Republicans in the House of Representatives who passed a tough border security bill that would further criminalise illegal presence in the United States. They are adamantly resisting the guest-worker programme.
His backing for the proposed law is costing him support among some conservatives who view it as a type of amnesty for illegal immigrants - a characterisation Bush rejected.
"We must face the reality that millions of illegal immigrants are already here," Bush said. "They should not be given an automatic path to citizenship. This is amnesty, and I oppose it."
Bush also said newcomers have a duty to assimilate into American society by learning English and US history.
The Minuteman Civil Defence Corps, a citizens' volunteer border patrol group, called Bush's plan "nothing more than a political ploy".