Calexico, California (CNN) President Donald Trump's Homeland Security secretary declined to explain what the President meant by "breeding concept" in a tweet Wednesday morning, saying he was calling attention to a "very serious problem."

Asked here during a visit to the border fence replacement project what Trump meant and whether the tweet had racial overtones, Kirstjen Nielsen said she couldn't speak "to a particular meaning of the tweet."

"Generally from where I stand, we have immigration laws, we need to enforce them no matter who is breaking those laws," Nielsen told CNN in an exclusive interview. "I think overall he's talking about a very serious problem that we have with sanctuary cities who are turning their back on federal laws and making it that much more difficult and dangerous for us to do our jobs to protect the American people."

Trump tweeted Wednesday morning about California, which has passed a state law limiting cooperation with federal immigration enforcement.

"There is a Revolution going on in California. Soooo many Sanctuary areas want OUT of this ridiculous, crime infested & breeding concept. Jerry Brown is trying to back out of the National Guard at the Border, but the people of the State are not happy. Want Security & Safety NOW!" Trump tweeted.

There is a Revolution going on in California. Soooo many Sanctuary areas want OUT of this ridiculous, crime infested & breeding concept. Jerry Brown is trying to back out of the National Guard at the Border, but the people of the State are not happy. Want Security & Safety NOW! — Donald J. Trump (@realDonaldTrump) April 18, 2018

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