The latest statement reinforced his hardline stand despite an about-face on family separations that has seen more than 500 children reunited with relatives.
Trying to stanch the flow of tens of thousands of migrants from Central America and Mexico arriving at the southern border every month, Trump in early May had ordered that all adults crossing illegally would be arrested, and their children held separately as a result.
After images of children in chain-link enclosures sparked domestic and global outrage, the president ended the separation practice but has continued his hardline talk on immigration.
He sees the issue as crucial ahead of midterm congressional elections in November.
â€œWe cannot allow all of these people to invade our Country,â€ Trump saidÂ SundayÂ on Twitter.
â€œWhen somebody comes in, we must immediately, with no Judges or Court Cases, bring them back from where they came,â€ said Trump, suggesting they be handled without the due legal process guaranteed for â€œany personâ€ by the US Constitution.
Nearly all of the arriving families have officially requested asylum.
â€œOur system is a mockery to good immigration policy and Law and Order,â€ said Trump, who has repeatedly tried to link immigrants with crime.
His remarks came after the Department of Homeland Security released its first official data since Trump ended the family separations on Wednesday.
It said 522 children separated as part of â€œzero toleranceâ€ have been reunited with their families, but another 2,053 separated minors remained in the care of the US Department of Health and Human Services as ofWednesday.
â€œThe United States government knows the location of all children in its custody and is working to reunite them with their families,â€ the department said in a statement lateSaturday.
Fleeing from impoverished Central America, the arrivals say they are seeking a better life and also a refuge from criminal gangs terrorizing their region, which has one of the worldâ€™s highest murder rates.
Central American migrants deported from the US without their children have spoken of their anguish at seeing families split under the â€œzero toleranceâ€ approach.
Ever Sierra, deported after trying to enter the US, told AFP he planned to try again in a few days.
He arrived back in Honduras with his eight-month-old daughterâ€™s shoes hanging from his backpack. She was being held in a detention center in McAllen, Texas, along with her mother.
Benjamin Raymundo, a 33-year-old deported back to Guatemala, told AFP he left his home country in April with his son Roberto, aged five, but the pair were separated when immigration officers in California stopped them. (NAN)