US President Bush signed a bill to build 700 miles of border fence between the U.S. and Mexico.

"We have more to do," Bush said during a brief signing ceremony at the White House.

Calling the border fence legislation "an important step toward immigration reform" Bush added that the US "must reduce pressure on our border by creating a temporary-worker plan. We must face the reality that millions of illegal immigrants are already here."

Mexican President-elect Felipe Calderon, who has been sharply critical of the proposed fence.

About 300 miles of the structure would divide Texas and Mexico, with 400 miles stretching along the border of other Southern states. The double-layered barrier is estimated to cost anywhere from $2.2 billion to $9 billion.

Bush had been pushing an immigrant guest-worker program and an opportunity for citizenship for most of the nation's estimated 12 million illegal immigrants.

"Ours is a nation of immigrants. We're also a nation of law. Unfortunately, the United States has not been in complete control of its borders for decades, and therefore illegal immigration has been on the rise," Bush said.