"If you cross the border unlawfully, then we will prosecute you".
Kirstjen Nielsen, the homeland security secretary, sent the order to border agents on Friday, according to two people familiar with the directive.
In Sessions' prepared remarks, he pledged his department would take up as numerous referrals from DHS "as humanly possible".
In one recent example of how the policy works, a woman who is suspected by the Justice Department of being part of the migrant caravan that was caught in a group north of the border was charged with illegal entry to the U.S. and separated from the four young children who were with her.
According to the report, families that are detained for illegal crossings have typically been kept together in the past.
Sessions also said that families who illegally cross the border may be separated after their arrest, with children sent to juvenile shelters while their parents are sent to detention facilities.
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.
The move comes amid intense attention to the caravan - an annual journey of mostly women and children fleeing what they say are unsafe situations at home through Mexico to make asylum claims in the US.
President Donald Trump described the caravan as proof that more must be done to secure the border with Mexico, including building a wall, while Sessions called it "a deliberate attempt to undermine our laws and overwhelm our system".
The Department of Justice says Sessions will hold a news conference Monday afternoon in San Diego.
Sessions said the US will put in place a "zero tolerance policy for illegal entry on our southwest border".
Sessions made clear his intention isn't to help caravan migrants navigate the labyrinthine process of asylum-seeking with greater ease-his Department of Justice has already filed criminal charges for illegal entry against 11 asylum seekers believed to be members of the caravan.
Thirty-five federal prosecutors and 18 immigration judges have been re-directed to the border to handle immigration cases.
"In this administration, we know whose side we are on", Sessions said Monday.