– About 900 National Guard troops have been deployed to the U.S.-Mexico border to help carry out President Donald Trump's border security agenda, administration officials said Monday.

An estimated 250 troops have been deployed in Arizona, 60 in New Mexico and roughly 650 in Texas, said Lt. Gen. Daniel R. Hokanson, vice chief of the National Guard Bureau. Trump has said he wants 2,000 to 4,000 troops deployed.

Under the current mission, the Guard troops will not be involved in any law enforcement activities and will not come into contact with undocumented immigrants who are caught crossing the border, said Border Patrol Chief Ronald Vitiello.

"Border security is and will remain a civilian operation," Vitiello said.

If the mission changes in the future, it will not involve the Guard being asked to help with immigration enforcement, Vitiello said.

"That's a red line," he said. "They will not be given assignments that require them to do law enforcement work."

Trump said in early April that he would work with border-state governors to deploy troops along the U.S.-Mexico border to counter rising levels of illegal immigration and other threats, including drug smuggling.

Illegal border traffic has risen sharply in recent months but is still considerably lower than in recent years. The Border Patrol apprehended 37,393 undocumented immigrants in March, compared with 26,662 in February, the Homeland Security Department said.

California National Guard troops are not involved in the mission, Vitiello said. The state's Democratic governor, Jerry Brown, told the administration Monday that he will not agree to contribute troops because the administration's requested services — including surveillance and engineering work — are closely related to immigration enforcement.

Brown said last week that he would agree to deploy 400 troops as long as they were not involved in immigration enforcement duties. The administration asked him to send 237 troops, officials said.

The Trump administration and the state of California have been engaged in an escalating battle over immigration issues such as the president's bid to strip federal funds from so-called sanctuary cities, counties and states that do not cooperate with federal immigration authorities. Brown is scheduled to speak to the National Press Club on Tuesday morning, and will likely discuss immigration issues.

Vitiello said the tasks asked of the California troops closely resembled the ones agreed to by the other border governors, all of them Republicans. He said California could play a role in the future because the National Guard's mission is likely to expand before the Department of Homeland Security achieves what it considers "operational control" of the border.

All of the National Guard troops will assist with ground and aerial surveillance, road and vehicle support, and administrative and mechanical support, said Vitiello. Whether they carry firearms is up to each governor.

Vitiello said the presence of the Guard would allow DHS to continue with other aspects of its mission — including construction of various sections of Trump's border wall — that will allow it to achieve operational control, which he called "a very high standard" for border security.

However, the troops will not help build the wall, Vitiello said.