The towering metal fence that divides the United States and Mexico at the edge of the Pacific Ocean has an imposing new feature: rows of razor wire.

The wire -- you've seen it images from battlefields and prisons -- is the most visible result of the $210 million military deployment along the U.S.-Mexico border, creating an imposing sight for Central American migrants gathering in Tijuana, Mexico, their eyes on the U.S.

At least 3,000 migrants have arrived in Tijuana, a border city across from San Diego, in the past two weeks, as Fox News previously reported. The federal government estimates the number of migrants could grow to 10,000 in the coming weeks or months. Tijuana residents have not fully embraced members of the caravan. During the weekend, about 500 residents protested the migrants by marching to an outdoor sports complex where the migrants were camped out, with protesters chanting: “Get out!” and “We don’t want you here!”

Border agents have arrested dozens of migrants trying to illegally cross the border. About 5,800 active-duty troops dispatched to the border to deal with the migrant crisis started coming home this week. On Monday, a judge barred the president from enforcing a ban on asylum for those who cross the U.S. border illegally – a decision the administration said would cause “countless illegal aliens to pour into our country.”

Homeland Security Secretary Kirstjen Nielsen said Wednesday an appeal will be filed on the decision. Speaking in San Diego, Nielsen said it would be filed as soon as possible. She said the ruling was “dangerous” for the country.

White House Press Secretary Sarah Huckabee Sanders railed against the California court that issued the ruling. The judge sided with civil rights groups that filed a lawsuit on the new regulations saying immigration law clearly states any migrant can claim asylum no matter how they arrived in the country. Sanders said the administration would take “all necessary action” to defend the decision to refuse asylum to people who cross between borders.

Soldiers and contractors have been installing different versions of the sharp wire barriers all along the border — on the fence, at ports of entry and on the banks of the Rio Grande in Texas, to name a few.

“This is what it really looks like - no climbers anymore under our administration,” President Donald Trump tweeted out, along with a photo of the fortified section in Southern California.

Military officials said they have more than 150 miles of wire available to string up at strategic locations.

The wire is called concertina wire, named after the musical instrument and its expanding and contracting bellows. It resembles barbed wire that’s commonly used to corral cattle, but it comes in flat coils that can expand quickly like its namesake, making it easier to store, transport and install.

It has been the wire of choice for militaries around the world since World War I.

Made in factories today using galvanized steel, concertina wire consists of sharp flat blades. It’s used to secure businesses, prisons and other government installations.

Critics have blasted Trump’s military deployment at the border as an expensive political stunt — and question why active duty troops are needed for tasks that contractors could perform.

According to figures reported to Congress on Tuesday, the Pentagon estimates the cost of the military’s mission on the U.S.-Mexico border will be about $210 million under current plans. That includes money for active-duty troops and National Guard troops.

What’s not clear is how much has been spent on materials like razor wire.

A ton of razor wire can go for several hundred dollars, while a single roll can be bought online for as little as $30.

An initial 22 miles of concertina wire was shipped to the border when the first troops began to deploy to California, Arizona and Texas a month ago. Military officials said at the time they had more than 150 miles of wire available.

It’s clear it won’t be strung along every mile as it would take 13 times that amount to span the U.S.-Mexico border. The border stretches about 1,950 miles from the Pacific Ocean to South Texas where the Rio Grande dumps into the Gulf of Mexico.

It’s up to U.S. Customs and Border Protection to tell the military where to put the wire. U.S. Defense Secretary Jim Mattis has said the short-term objective has been to get enough wire and other barriers in place as requested by border officials.

A Defense Department spokesman, Army Col. Robert Manning, told reporters earlier this week that more than four miles' worth had been installed by soldiers in Arizona and California.

The Associated Press contributed to this report.