AUSTIN, TX - APRIL 06: Texas National Guard helicopters traveling to the Texas-Mexico border prepare to fly on April 6, 2018 in Austin, Texas. Brigadier General Tracy Norris announced during a press conference that the Texas National Guard will immediately deploy an expected 250 peronnel with supporting aircraft, vehicles and equipment within 72 hours. (Photo by Drew Anthony Smith/Getty Images)
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AUSTIN, TX - APRIL 06: Texas National Guard helicopters traveling to the Texas-Mexico border prepare to fly on April 6, 2018 in Austin, Texas. Brigadier General Tracy Norris announced during a press conference that the Texas National Guard will immediately deploy an expected 250 peronnel with supporting aircraft, vehicles and equipment within 72 hours. (Photo by Drew Anthony Smith/Getty Images)
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(CNN) —  

California Gov. Jerry Brown has agreed to send more National Guard troops to the border with Mexico, but said Wednesday they will focus on fighting crime and not enforce policies of Washington.

“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,” Brown said. “And the California National Guard will not be enforcing federal immigration laws.”

Brown said 400 troops will augment a current mission that fights transnational crime.

The location of Guard personnel – and number along the border, the coast and elsewhere in the state – will be dictated by the needs on the ground, a spokeswoman for the governor said.

It will take some time before Guard troops can be deployed. It has been one week since California received the current request. In other operations it has taken several months to get troops where they are needed.

One ongoing operation has 55 Guard troops at the 140-mile border and 250 deployed statewide.

US Department of Homeland Security Secretary Kirstjen Nielsen tweeted that she is “glad to have all four border governors working with us and the US National Guard to secure the border.”

She added that “partnership [with] governors is vital to our nation’s success.”

The governor’s statement said 15% of border apprehensions occur in California.

“I agree with the Catholic bishops who have said that local, state and federal officials should ‘work collaboratively and prudently in the implementation of this deployment, ensuring that the presence of the National Guard is measured and not disruptive to community life,’ ” Brown said.

State leaders in California have made no secret of their opposition to many of the Trump administration’s moves on immigration. And for days, leaders there had been tight-lipped about whether they would deploy National Guard troops.

A spokesman for the California Army National Guard initially said the administration’s request would be promptly reviewed.

“We look forward to more detail, including funding, duration and end state,” Lt. Col. Tom Keegan said.

US Defense Secretary James Mattis has authorized the deployment of up to 4,000 National Guard personnel through September 30.

Last week Defense Department officials said that without approval from Mattis, troops would not perform law enforcement activities or interact with migrants or others who are detained. Officials say the troops will only carry weapons in “circumstances that might require self-defense.”

In past border operations, officials have said National Guard troops cannot:

• Arrest people

• Seize drugs

• Conduct searches

On Tuesday, a captain with the Texas National Guard told CNN during a visit to an observation post on the border there that troops act as a physical deterrent and as extra sets of eyes and ears for Border Patrol agents.

CNN’s Tal Kopan and Catherine Shoichet contributed to this report.