California has rejected the federal government's initial plans for National Guard troops to the border because the work is considered too closely tied to immigration enforcement, two U.S. officials told The Associated Press.

The state informed federal officials it will not allow its troops to fix and repair vehicles, operate remotely-controlled surveillance cameras to report suspicious activity to the Border Patrol, operate radios and provide 'mission support,' which can include clerical work, buying gas and handling payroll, according to officials with knowledge of the talks who spoke condition of anonymity because they were not authorized to discuss the matter.

California Gov. 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.

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.

Soldiers from the Texas Army National Guard keep watch on the banks of the Rio Grande, where some of them will be armed where necessary (pictured Wednesday)

A National Guard troop watches over Rio Grande River on the border in Roma, Texas on Wednesday. The deployment of National Guard members to the U.S.-Mexico border at President Donald Trump's request was underway Tuesday with a gradual ramp-up of troops under orders to help curb illegal immigration

But the Democratic Brown conditioned his support by insisting that California's troops have nothing to do with immigration enforcement.

He was not specific about jobs his troops would or would not perform or how he would distinguish between immigration-related work and going after criminal gangs and drug and gun smugglers.

California National Guard spokesman Lt. Col. Thomas Keegan said Monday that the state was awaiting a formal response from the administration and had no additional details beyond the governor's proposed agreement released last week that includes a ban on immigration enforcement.

Evan Westrup, a spokesman for the governor, did not immediately answer detailed question about California's rejection of specific guard duties.

Talks between U.S. and California officials about the duties the California troops would perform soured Friday and over the weekend after state authorities told federal officials that they would not participate in vehicle maintenance and the other jobs outlined for an initial phase across the border in California, Arizona, New Mexico and Texas, the U.S. officials said.

The other border-state governors - all Republicans - have openly embraced Trump's plans.

Ron Vitiello, the acting deputy commissioner of US Customs and Border Protection, said Brown had declined the initial roles put forward for Guardsmen.

'The governor has determined that what we have asked for so far is unsupportable,' Vitiello told reporters.

'We've made this refined request, it's gone through the process and then we've got a signal from the governor that he is not participating.'

Deputy Assistant Defense Secretary Bob Salesses said the initial request envisioned sending 237 Guardsmen to two main crossing areas in Southern California, where they would have conducted maintenance, clerical assistance and helped with heavy equipment operations, among other tasks.

'The California National Guard has indicated that they will not perform those missions as we know them to be right now,' Salesses said, though he noted that conversations were ongoing.

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Vitiello, too, suggested that the state's Guard might ultimately be used in other roles, including possibly cargo inspection.

'We will have other iterations,' Vitiello said.

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.

The order would eventually see about 4,000 Guardsman along the border, which spans four US states.

So far about 960 have arrived, officials said. Texas has seen the biggest deployment, with 650 sent to the border, while Arizona has dispatched 250, and New Mexico about 60.

Vitiello said Guardsmen would most likely not be armed, but individual states might allow the carrying of a weapon in certain missions.

California is at the forefront of what opponents call the 'Resistance' to Trump's administration, with the heavily Democratic state suing the federal government over numerous issues, including the rollback of environmental regulations.

Several cities including Los Angeles are 'sanctuary cities' that require local law enforcement agencies not to tell federal agents about residents' legal status.

US Attorney General Jeff Sessions has sued the state over three statutes that support cities and counties that refuse to hand over unauthorized immigrants to federal immigration authorities for prosecution or expulsion.