Bush has signed into law a controversial bill authorising construction of a new fence along the US-Mexico border.
The fence will run 1,100 kilometres along a third the US border with Mexico, shining the spotlight on illegal immigration 12 days ahead of US legislative elections.
At a White House bill-signing ceremony, Mr Bush said the law would make the frontier more secure and is an important step toward immigration reform.
"Unfortunately, the United States has not been in complete control of its borders for decades, and therefore illegal immigration has been on the rise," he said.
"We have a responsibility to address these challenges. We have a responsibility to enforce our laws. We have a responsibility to secure our borders. We take this responsibility seriously."
Mexico anger
A spokesman for Mexican President Vicente Fox said that building a fence along the border would not solve the problem of illegal immigration.
"We do not believe that walls can solve these problems. An integrated immigration reform system that allows for orderly, legal immigration while respecting human rights is the only solution," said Mr Fox’s spokesman Ruben Aguilar.
The Mexican spokesman charged that the decision, made ahead of the November 7 US legislative elections, was politically motivated. He predicted that the barrier would never be completed because of insufficient funding.
On October 4, the US president had signed a bill earmarking some $US1.2 billion ($A1.57 billion) in funding for the fence, in a bid to stanch the steady flow of illegal immigrants into the United States.
Mexico had warned the barrier would damage bilateral relations.
However the $US1.2 billion ($A1.57 billion) approved by Bush fell well short of estimates to build fencing along the porous southern US border.
The fence's cost has been estimated at up to $US6 billion ($A7.85 billion), while Senate Democratic opposition leader Harry Reid put the price tag at some $US8 billion ($A10.47).