The Department of Justice is cracking down even further on so-called sanctuary cities, saying that cities with such policies are not eligible for a federal assistance program used to help fight violent crime.

According to Attorney General Jeff Sessions, four cities — Albuquerque, N.M., Baltimore, Md., San Bernardino, Calif., and Stockton, Calif. — have expressed interest in the department's Public Safety Partnership, or PSP, program.

The initiative was launched in June in 12 cities that the Justice Department said needed "significant assistance" in combating "gun crime, drug trafficking and gang violence."

Now, in letters to those cities, which limit cooperation with the federal government when it comes to immigration law, Acting Assistant Attorney General Alan Hanson tells the police chiefs they must show a "commitment to reducing violent crime stemming from illegal immigration" in order to be added to the PSP program.

The four cities must prove to the Justice Department by Aug. 18 that they will give federal immigration agents access to jails to question immigrants, as well as provide 48 hours notice to the Department of Homeland Security regarding the release date and time of someone who has been flagged for violating federal immigration law.

The cities must also show they do not block communication between local police and federal immigration agents.

"By taking simple, common-sense considerations into account, we are encouraging every jurisdiction in this country to cooperate with federal law enforcement," Sessions said in a statement. "That will ultimately make all of us safer — especially law enforcement on our streets."

This move is Sessions' latest is his latest to make good on promises to crack down on immigration. Bolstering public safety is also a top priority for Sessions.

Last week, Sessions told jurisdictions that Immigration and Customs Enforcement agents must be able to enter prisons or jails, or Edward Bryne Memorial Justice Assistance Grants — or JAG grants — are in danger of being pulled.