By Randall Mikkelsen

WASHINGTON, Oct 22 (Reuters) - The Bush administration waived environmental and other laws on Monday and ordered construction to proceed on an environmentally sensitive section of the border fence between the United States and Mexico.

U.S. Homeland Security Secretary Michael Chertoff issued the waiver, which bypasses a federal judge's temporary restraining order against building the 1.5-mile (2.4-km) section of fence in a natural conservation area in Arizona.

Chertoff acted because any further delay would present "unacceptable risks" to security, the Homeland Security Department said. He acted on authority of various immigration and security laws.

"We need to move forward on building this fence," department spokeswoman Laura Keehner said.

The environmental groups Defenders of Wildlife and the Sierra Club had sought to block construction on grounds the fence's environmental impact had not been sufficiently studied, and a federal judge on Oct. 10 temporarily blocked further construction.

The fence is being built in southeastern Arizona in the San Pedro Riparian National Conservation Area, which the environmental groups describe as unique and biologically diverse, with more than 250 species of migratory birds having been recorded there.

The fence is part of 380 miles (612 km) of border fencing the department has committed to build by the end of 2008.

The department said the conservation area has also been an increasingly popular corridor for illegal immigration. It said there were more than 19,000 apprehensions and 21,000 illegal entries in the area in fiscal 2007, which ended last month. Fourteen immigrants died in the "inhospitable" surroundings during the period, it said.

The immigrants also cause environmental damage due to trash, soil erosion and wildfires caused by campfires, it said.

A statement by the two environmental groups accused Chertoff of bypassing environment laws to push through a "destructive, ineffective plan."

"Bulldozing the San Pedro Riparian National Conservation Area and our important environmental protections is not necessary to manage the border," said Sean Sullivan with the Sierra Club's southeastern Arizona chapter.

Chertoff had twice before exercised his waiver authority to complete construction on other sections of border fencing, Keehner said.

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