The policy, which the official said was signed on Friday, formalizes plans that have been under discussion for more than a year.
Sessions' visit to Southern California comes about a week after hundreds of Central American migrants traveled to the USA border seeking asylum following a monthlong caravan through Mexico.
"Let it be clear: the Department of Homeland Security's official policy is now to separate families - including mothers from their young children - when they seek assistance at the border between ports of entry", Thompson said in a statement.
Shortly after Trump took office, pledging a hard line against illegal immigration, border arrests fell.
"Illegal immigration must end!" he tweeted on Friday. The Wall Street Journal, citing the Department of Health and Human Services, reported earlier Monday that 700 children had been separated from their families since October of past year.
The policy would not apply to asylum seekers who come to an official port of entry to the United States without paperwork - those individuals would only be placed into immigration proceedings.
Trump Urges West Virginians Not to Vote for Don Blankenship
Manchin has been a top GOP target since at least November 2016, when Trump won the state with 68.7 percent of the vote. Moore's loss narrowed Republicans' Senate majority to 51-49.
Immigration advocates say that family separations for criminal prosecutions and other circumstances have already been happening for months.
Sessions' so-called "zero tolerance policy" raises the possibility that immigration authorities could separate parents from their children whenever a family is apprehended attempting to cross the border.
Thomas Homan, U.S. Immigration and Customs Enforcement's acting director, echoed Sessions, saying the policy will lead to more prosecutions, including parents who must be separated from their children.
Attorney General Jeff Sessions gestures as he speaks at the California Peace Officers' Association 26th Annual Law Enforcement Legislative Day on March 7, 2018, in Sacramento.
In prepared remarks released to the media at an event in Arizona on Monday, Sessions said the government would also go after immigrants who pay smugglers to bring children across the border.