(Adds reaction from watchdog group)

By Susan Heavey

WASHINGTON, Aug 1 (Reuters) - The United States will waive environmental rules so extra barriers can be built to bar illegal immigrants from crossing the border with Mexico near San Diego, the Department of Homeland Security said on Tuesday.

The projects, on about 15 miles (24 km) of the frontier extending east from the Pacific Ocean, are part of President Donald Trump's planned wall between the United States and Mexico.

The wall was a centerpiece of Trump's 2016 presidential campaign but Democrats and many Republicans have questioned its effectiveness. Immigration advocacy groups argue it would not stem illegal border crossings.

Government estimates put the cost of the wall at about $21 billion and Trump has said that Mexico will eventually pay for it. Mexico has refused to pay.

The DHS said the area near San Diego is one of the busiest U.S. border sections with a high volume of illegal entries and that there was an immediate need to improve the infrastructure.

Environmental impact studies generally are required under federal law for building on public lands. While the waiver will eliminate a study, the department said it will remain mindful of cultural and environmental impacts.

Roads, lights, cameras and sensors will be installed in addition to extra barriers, the department said. The project's start date was not immediately known and the DHS statement did not mention the cost.

The DHS statement said Border Patrol agents arrested more than 31,000 "illegal aliens" and seized more than 9,000 pounds (4,000 kg) of marijuana and 1,317 pounds (597 kg) of cocaine in the San Diego area last year.

According to a memo published by the Congressional Research Service in January and seen by Reuters, a 2005 law gives Homeland Security broad authority to waive any law that could impede expeditious construction of barriers and roads.

The memo was sent to members of Congress who questioned the feasibility of building a southern border wall.

The U.S. House of Representatives last week approved a $68 billion increase in military spending next year with legislation that also provided funds for the border wall.

American Oversight, a watchdog group that has sued the Trump administration on a range of issues, said the DHS statement was a disturbing sign that Trump will "barrel ahead" with building the wall, no matter the cost to taxpayers or the environment.

"Given the widespread skepticism towards the effectiveness of the border wall by leaders in both parties ... effective safeguards are more important than ever," American Oversight's executive director, Austin Evers, said in a statement.

Trump's 2018 budget proposal also calls for 32 miles (51 km) of new wall in Texas' Rio Grande Valley, where the 2,000-acre Santa Ana National Wildlife Refuge is located. That has worried environmental activists, who note the refuge is home to 400 bird species and a dwindling population of federally protected ocelots. (Additional reporting by Julia Edwards Ainsley; Writing by Gina Cherelus; Editing by Bill Trott and James Dalgleish)

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