Deputy Assistant Secretary of Defense for Homeland Defense Integration Robert G. Salesses says there is no estimate for the operation's cost, which is funded by the USA government.
Jerry Brown elicited rare and effusive praise from President Donald Trump last week after he pledged 400 troops to the Guard's third large-scale border mission since 2006.
California has rejected a preliminary federal plan for the state's National Guard troops deployed to the border because it entails too much support for immigration enforcement, according to US officials.
Brown has been the only holdout among border state governors, as Texas, New Mexico and Arizona - all led by Republicans - moved quickly to send personnel.
The governor's commitment allowed Trump to boast support from all four border-state governors and helped put the president above the lower end of his threshold of marshaling 2,000 to 4,000 troops that he wants as a border security mission to fight illegal immigration and drug trafficking.
Acting U.S. Customs and Border Protection Deputy Commissioner Ronald D. Vitiello says troops that work at border crossings will perform cargo inspections that are not viewable by the general public. Earlier this month, President Donald Trump and Protection Secretary James Mattis licensed Upto 4,000 National Guard staff that will assist the Department of Homeland Protection secure the border from several southwestern US states.
Discussions between Sacramento and federal officials hit a roadblock over the weekend, when state authorities refused to agree to let guardsmen do vehicle maintenance and other jobs outlined in an initial federal plan, according to the AP's report. According to one USA official, the California Guard has suggested assigning about 40 troops to marijuana eradication across the state.
Trump's Lawyer To Appear In Federal Court Over FBI Seized Materials
The next phase is a battle over the procedures involving who may review the materials seized by FBI investigators last week. All the same, this legal back and forth over the potential review, she added, is "highly, highly unusual".
Brown was clear last week that California troops will help go after drugs, guns and criminal gangs, but not immigrants.
Vitiello said the federal government wanted 237 troops for work in two parts of the state that California "has indicated they will not perform", but he emphasized that California may participate in other ways that must still be worked out.
Texas, Arizona and New Mexico have already agreed to the Trump administration's request for National Guard support.
Trump this month said he would send thousands of National Guard troops to the southern border, where they could remain until a border wall is constructed.
"We are anticipating additional requirements, and we got a signal from California that they are interested in improving border security", Vitiello said.
Speaking to reporters in Washington, Lt. Gen. Daniel R. Hokanson said Monday that about 650 are deployed in Texas, with close to 250 in Arizona and more than 60 in New Mexico.