PHOENIX — Construction crews broke ground Thursday on a small portion of the $664 million border fence project in the Arizona desert that is funded through President Donald Trump’s national emergency declaration.

Crews plan on installing 30-foot steel fencing to replace older barriers on two miles in Organ Pipe Cactus National Monument, next to the official border crossing known as the Lukeville Port of Entry.

The project is funded through the Defense Department. Use of the department’s money was previously frozen by lower courts while a lawsuit proceeded. But the U.S. Supreme Court last month cleared the way for the use of about $2.5 billion.

A border wall was a major milestone of the president’s election campaign. Congress this year allocated $1.4 billion, but the president wanted much more. He declared a national emergency in February and faced legal challenges for plans to build dozens of miles of fencing almost immediately.

In Arizona, environmentalists have sued over some of the construction contracts, saying the government unlawfully waived dozens of laws to be able to build on protected lands. They say a wall — and construction for it — would be detrimental to wildlife habitat. The case before federal court is pending.

Thursday’s groundbreaking was on a portion of fencing that stretches west from the Lukeville Port of Entry, an official border crossing, Border Patrol spokesman Jesus Vasavilbaso said. That’s the port many Arizona residents pass on their way to the Mexican beach destination commonly known as Rocky Point.

Crews there have been clearing out brush and removing the old fencing in preparation for the installation of the bollards all week.

Construction is expected to take about 45 days, according to court documents filed last week. The government plans to then tackle two other projects in Arizona, including nearly 40 miles of fencing in other parts of Organ Pipe Cactus National Monument and in the Cabeza Prieta National Wildlife Refuge, and a smaller project at the San Pedro Riparian National Conservation Area. The two other projects are slated for early October.

The projects come as immigrant apprehensions have fallen sharply over the last two months. The high summer temperatures usually result in fewer people attempting to cross, and Mexican authorities have been clamping down on immigrants traveling north through the country.

But people still try their hand in this rugged, extremely hot desert area where in June a 6-year-old Indian girl died after attempting to walk the desert with her mother, who was uninjured.