Updated Tuesday with Trump's attack on Brown

WASHINGTON -- California has so far refused to provide National Guard troops for a planned border surge, amid concerns they would be involved too closely with immigration enforcement, officials said Monday.

That puts a dent in the deployment ordered this month by President Donald Trump. On Tuesday, he lashed out at Gov. Jerry Brown, asserting on Twitter that he is "not looking for safety and security along their very porous Border."

Looks like Jerry Brown and California are not looking for safety and security along their very porous Border. He cannot come to terms for the National Guard to patrol and protect the Border. The high crime rate will only get higher. Much wanted Wall in San Diego already started! — Donald J. Trump (@realDonaldTrump) April 17, 2018

The nation's Border Patrol chief said the deployment will last until "operational control" is achieved -- a goal that could mean a lengthy if not indefinite deployment. Trump himself said he wants 2,000 to 4,000 troops until Congress provides funding to construct a border wall.

"The governor has made that choice.... We're grateful for the other border states for stepping up," said Ronald Vitiello, acting deputy commissioner of Customs and Border protection, and chief of the Border Patrol.

Pentagon and homeland security officials said Monday that California Gov. Jerry Brown, a Democrat, has rejected requests for guard troops in the roles sought by the Border Patrol. He pledged last week to provide 400 troops on condition they not be used to support immigration enforcement.

California National Guard spokesman Lt. Col. Thomas Keegan called it "inaccurate" to say that Brown has rejected the request for troops. Rather, he said by email, the Trump administration has yet to respond to California's request for assurance the Guard would only be used to help fight transnational gangs, human trafficking, and smuggling of guns and drugs.

Brown is willing to order 400 troops to active duty on those conditions, as he wrote in a letter April 11 to Defense Secretary Jim Mattis and Homeland Security Secretary Kirstjen Nielsen.

"But let's be crystal clear on the scope of this mission. This will not be a mission to build a new wall. It will not be a mission to round up women and children or detain people escaping violence and seeking a better life. And the California National Guard will not be enforcing federal immigration laws," Brown wrote. "Here are the facts: there is no massive wave of migrants pouring into California."

Vitiello said the Border Patrol is retooling its requests and indicated optimism that California might be willing to provide help with cargo inspections at border crossings -- a role that would keep them from public view and could reduce the chances of contact with people crossing illegally.

Operation Guardian Shield

According to federal officials, guardsmen deployed as part of the latest mission -- dubbed Operation Guardian Shield -- will help fix and maintain vehicles, aircraft and roads.

They'll replace Border Patrol personnel who clear brush, operate surveillance cameras and perform clerical work. They hope to have Guard units performing ground and aerial surveillance missions in all four border states. Texas Guard troops already do such work and Arizona's Guard said last week that it will also fill those roles.

"The California National Guard will not perform those missions," said Robert Salesses, a deputy assistant secretary of defense. "We are in continuing dialogue."

Roughly 900 troops have been deployed so far in response to Trump's request, Lt. Gen. Daniel Hokanson, vice chief of the National Guard Bureau, told reporters Monday: about 650 in Texas, 60 in New Mexico, and 250 in Arizona.

He emphasized that National Guard personnel will not perform any law enforcement function. Nor will they be placed in harm's way, a point echoed by Vitiello and Robert Salesses, a deputy assistant secretary of defense.

1 / 3Texas Gov. Greg Abbott, left, talks with leaders of the Texas National Guard and the U.S. Border Patrol at the Texas National Guard Armory on Thursday, April 12, 2018, in Weslaco. Governor Abbott announced that there will be an increase of 1000 Texas National Guard on the southern border with Mexico. (Joel Martinez/The Monitor via AP)(Joel Martinez / AP) 2 / 3Protestors chant as Texas Gov. Greg Abbott visits the Texas National Guard Armory on Thursday, April 12, 2018, in Weslaco. Governor Abbott announced that there will be an increase of 1000 Texas National Guard on the southern border with Mexico. (Joel Martinez/The Monitor via AP)(Joel Martinez / AP) 3 / 3A Border Patrol officer talked with a member of the Texas National Guard along the Rio Grande in Starr County, Texas, April 10, 2018. As the National Guard begins a controversial mobilization on the southern border, soldiers are trying to fulfill two seemingly contradictory missions: standing out, and blending in. (Lynsey Addario/The New York Times) (LYNSEY ADDARIO / NYT)

"National Guard members will not -- will not -- enforce law. They will not perform law enforcement functions and they will not be placed in direct contact with personnel coming to the border," said Salesses.

Vitiello said requests to all four states involved similar missions and functions - "basically operational support things."

He and other federal officials emphasized that no guardsmen will be "required" to be armed. But some may carry weapons, depending on their assignments. They didn't provide details on how many of the troops currently deployed carry firearms.

"None of the roles that we've asked for are going to require soldiers to do law enforcement work," Vitiello said, adding, "That's a red line. They will not be given assignments that require them to do law enforcement work."

Vitiello described "operational control at the border" as the goal of the deployment, and called that "a very high standard." It requires a combination of more personnel and technology, and more fencing.

The elements including "situational awareness," the ability to stop border incursions, and respond quickly.

Critics say the deployment could be indefinite, and contend that it saps funds needed for other Pentagon missions, including upgrades and repairs of equipment and weapons systems. For now, the funding comes from the National Guard's training accounts.

Critics also cite concerns about militarizing the border, pointing to incidents in which armed troops have injured or killed civilians, including American citizens. In 1997, a U.S. Marine on a drug patrol shot and killed Ezequiel Hernandez, an 18-year-old American who was tending goats near the border at Redford, Texas, south of Presidio.

Lessons learned

Previous presidents have sent federal troops to shore up security and reinforce the Border Patrol.

Operation Jump Start lasted two years from 2006 to 2008. President George W. Bush ordered 6,000 troops to the border in support roles such as aviation and logistics, to free up Border Patrol agents to focus on catching and detaining immigrants in the country illegally. In 2010, President Barack Obama deployed 1,200 troops under pressure from Rick Perry -- who at the time was Texas' governor and is now Trump's energy secretary.

Vitiello said guard troops will be kept away from any "higher threat environment," acknowledging that in previous deployments, some Border Patrol agents were effectively assigned to keep an eye on guardsmen.

"That's a lesson learned from Jump Start," he said.