WASHINGTON — The administration is aiming to construct a wall along roughly half the U.S.-Mexico border, the nation's Border Patrol chief said Friday — a thousand-mile barrier that could fall well short of what President Donald Trump has demanded.

Congress has rebuffed Trump's demand for $25 billion for wall construction. The spending bill that Trump grudgingly signed a week ago includes $1.6 billion for border security, enough for just about 33 miles of new barrier plus about twice that much of replacement fencing.

Ronald Vitiello, chief of the Border Patrol and acting deputy commissioner of U.S. Customs and Border Protection, said a thousand miles of barrier "will make a major dent" in the nation's security challenges. But in a briefing with reporters, he conceded that he's not sure Trump has signed off on that target.

"We appreciate this down payment ... but it does not fully fund our needs in the most critical locations," he said.

Roughly 654 miles of the border is already fenced, out of 1,954 miles. Most of that was built under the 2006 Secure Fence Act, signed by President George W. Bush.

In recent weeks, the view from Trump and his administration has vacillated between frustration that Congress has refused to fund his wall and boasts that construction is well underway.

Trump was so unhappy about the sum Congress provided for border security in the $1.3 trillion spending deal that he threatened a veto, raising the specter of a government shutdown. He relented within hours and signed the measure March 23.

Vitiello said Friday that $25 billion would be enough to replace or upgrade existing fencing and add about 300 miles of new pedestrian barrier. Vehicle barrier accounts for 300 miles of current fencing. The rest is higher and intended to keep out individuals.

The president and his aides have also sought to blur the distinction between a "wall," a "wall system" and fencing.

Trump has been adamant that only a wall will suffice, though the descriptions he's offered of his vision have evolved. He has acknowledged for months that an opaque barrier isn't good enough, because border agents need to see what's on the other side to intercept and deter drug smugglers and immigrants trying to cross illegally.

On Thursday, he reiterated his commitment to building a border wall. "We've done prototypes all over, and we have something special happening," he told a crowd in Richfield, Ohio, during a speech on infrastructure.

On Wednesday, Trump tweeted out photos of a section of fencing under construction in Calexico, Calif., characterizing it as "the start of our Southern Border WALL!"

In fact, that 2-mile, $18 million section of 30-foot fence has been planned since 2009 — long before Trump became a candidate for president.

Great briefing this afternoon on the start of our Southern Border WALL! pic.twitter.com/pmCNoxxlkH — Donald J. Trump (@realDonaldTrump) March 28, 2018

Two hours after Trump's tweet, Homeland Security Secretary Kirstjen Nielsen added that: "This is just one of the wall projects @DHSgov will be constructing in the next year."

Vitiello defended Trump's characterization, saying the Calexico project replaces a "dilapidated" fence with a taller and stronger barrier.

The spending deal Trump signed into law includes provisions that chafe the president. The 2,232-page bill explicitly limits wall construction to "operationally effective designs" deployed by March 2017 — effectively ruling out the prototypes he just inspected.

"We're still considering what it means," Vitiello said.

Still, he said, the prototypes have been useful in providing lessons for future designs.

"We learned about what it takes to breach some of that structure," he said. "They were cut on and beat on and climbed on and dug into."

The law also bans construction in the Santa Ana National Wildlife Refuge in Hidalgo County — a win for environmentalists who have warned that roads and fences would destroy an important ecosystem.

"It's definitely a hiccup and it's going to be dealt with," Tyler Houlton, Homeland Security Department press secretary, said Thursday. "We're looking at regulatory changes. We need the ability to build the wall."

The semantics are important in terms of political bragging rights. Trump's insistence on a "wall" has left even many border security hawks exasperated, because technically, that term describes a solid barrier that isn't see-through.

As the Los Angeles Times has reported, the Calexico project involves bollards — posts placed close enough together to prevent people from passing through, but leaving enough space for Border Patrol agents to see the other side. Federal authorities had long referred to similar but shorter construction in the area as fencing.

"30 feet — that's a wall in my mind," Houlton said. "The wall will change operationally ... because different areas have different needs ... but it's definitely a wall."

Visiting the Cleveland area Thursday, Trump kept up the drumbeat for a wall.

"Drugs are flowing across borders. We need walls. We started building our wall. I'm so proud of it. We started. We started. We have $1.6 billion, and we've already started," he said. "You saw the pictures yesterday. I said, `What a thing of beauty.'"

And he dismissed carping about the shortage of funds, and speculation that he has tried to narrow the scope of the project or back down from his campaign vow in the face of fierce resistance in Congress, from both parties.

"People said, 'Oh, has he given up on the wall?' No, I never give up. We have $1.6 billion toward the wall, and we've done the planning. And you saw those beautiful pictures, and the wall looks good. It's properly designed," Trump said, though it was unclear what design he was referring to, since no design has been finalized or announced.

"We are building a really state-of-the-art, very, very efficient — have to be able to see through; it makes a lot of sense. You have to be able to see who is on the other side."

Trump vowed throughout the 2016 campaign to build a wall and to force Mexico to pay for it. Mexico has flatly rejected the idea and Trump has made no proposal for funding other than from U.S. taxpayers and federal revenue. In recent days, he has privately floated the idea of shifting funds from the Pentagon budget.

Budget experts say that's not legal. And defense hawks and Democratic critics have expressed dismay, suggesting a bait-and-switch tactic. The White House pressed Congress to boost the military budget in the $1.3 trillion "omnibus" deal, to achieve another of the president's goals — shoring up the nation's armed forces.

Contractors have built eight wall prototypes near San Diego, using specifications that include resistance to climbing, blasting and cutting. Trump inspected the samples March 13 during his first presidential visit to California.

"If we don't have a wall system, we're not going to have a country," he tweeted hours later as the White House trumpeted the visit with a video of Trump at the wall prototypes. "Congress must fund the BORDER WALL & prohibit grants to sanctuary jurisdictions that threaten the security of our country & the people of our country. We must enforce our laws & protect our people! #BuildTheWall"

The funds Congress has authorized for 2018 provide enough to build about 100 miles of fence or wall: 14 miles of replacement wall in San Diego, plus another 14 miles of secondary fencing; 20 new miles around Santa Teresa, N.M., just west of El Paso, with groundbreaking early in April; 4 miles of new fencing in El Paso; 25 miles of leveewall in Hidalgo County; and 8 miles of new "border wall system" in Starr County, including fencing, roads and lighting.