The new contingent of California Guard members being deployed could be posted at the border, the coast and elsewhere statewide, Brown said.
Brown said the deployment will happen pending review and approval of the federal government.
Trump administration officials have said that rising numbers of people being caught at the southern border, while in line with seasonal trends in recent years, require an immediate response.
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NBC News first reported on Wednesday that a sexual assault case from Spacey was presented for the district attorney's off ice. Prosecutors are reviewing a decades-old sex crimes case against disgraced actor Kevin Spacey .
The spokeswoman for President Donald Trump has welcomed the decision by California's Democrat governor to deploy 400 National Guard troops for a presidential request aimed at fighting illegal immigration and drug trafficking along the U.S. -Mexico border. Already at the state's 140.4-mile border with Mexico are 55 servicemembers on that mission.
California's "sanctuary" policies limiting cooperation with US immigration agents have been a target for Trump's ire, and in his most recent weekly address he called the state "a border-free zone where thousands of criminal aliens can roam free".
Arizona recently deployed its first 225 troops to its border.
At that time, he added, the federal government will determine whether it has the budget and whether the mission is necessary to continue.
Track and field: LP boys, BV girls win Howard-Monier titles
Chase Reijo swept the jumps for Hayward, soaring 20-0 to win the long jump and going 41-5 to take the triple jump . The Lady Bulldogs also received one point for finishing in sixth place in the both the 4 x 100 and 4 x 200 relays.
The governor stressed in his statement that the troops will not be used to "round up women and children or detain people escaping violence and seeking a better life".
He says none of the troops who are being deployed are required to carry guns but that they can do so if they feel the need to protect themselves. Brown added that "there is no massive wave of migrants pouring into California", saying that overall, immigration apprehensions on the border last year were as low as they've been in almost 50 years. "That's why the state and the Guard have long supported this important work and agreed to similar targeted assistance in 2006 under President Bush and in 2010 under President Obama".
And the governor insisted in his letter that Trump's public pronouncements on border threats were misguided - pointing to statistics that show the number of detained immigrants in California is at its lowest point in almost 50 years. Brown noted in his letter that "immigrant apprehensions on the border last year were as low as they've been in almost 50 years".
Saudi forces say its defence systems intercepted Yemeni rebel ballistic missile
The news agency said that Saudi Aramco was building a refinery in Jizan planned to produce 400,000 barrels of oil a day from 2019. Reports surfaced earlier that loud blasts had been heard in Saudi Arabia's capital Riyadh.