What are the 'sanctuary cities' that Trump opposes?
US President Donald Trump speaks to staff at the Department of Homeland Security in Washington, DC, on January 25, 2017, where he vowed to restore "control" of US frontiers by building a wall on the Mexican border (AFP Photo/Nicholas Kamm)

US President Donald Trump on Wednesday vowed that his administration would crack down on "sanctuary cities" in the US that protect undocumented immigrants.

Some 300 such cities, counties or states -- from New York to Los Angeles -- exist throughout the United States, and many of them have vowed since Trump's election to protect the estimated 11 million undocumented migrants living in the country.

- What is a 'sanctuary city'? -

The term refers to Americans cities, counties or states -- such as New York or California -- that protect undocumented immigrants from deportation by refusing to assist or cooperate with federal immigration officials. Such cities normally do not allow local police to inquire about the immigration status of people with whom they interact.

They also refuse to detain people brought to them by immigration authorities or to keep suspected undocumented immigrants in jail beyond their scheduled release date.

Los Angeles became one of the first sanctuary cities in 1979 after numerous faith communities began offering shelter to refugees fleeing conflict in Central America and who could not get asylum.

According to the Center for Immigration Studies (CIS), a nonprofit group, some 300 sanctuary cities exist across the country.

Jessica Vaughan, director of policy studies at CIS, said some of these cities, like New York, Chicago and San Francisco, have simply cut ties with federal immigration officials and refuse to cooperate with them.

- What do these cities now face? -

The decree signed by Trump on Wednesday threatens to withhold federal funds from cities that refuse to cooperate with immigration officials.

Such funds can represent a considerable chunk of a city's budget. According to CNN, New York alone stands to lose $10.4 billion in federal funds for social services and other programs.

Vaughan said the federal agency most likely to cut off funding is the Justice Department, which provides grants to local law enforcement agencies to, among other things, assist crime victims and prepare for terrorist attacks.

A showdown is expected between the Trump administration and local governments with sanctuary cities, which have been digging in for a fight.

Los Angeles last month pledged $10 million to provide legal assistance to immigrants facing deportation. Chicago Mayor Rahm Emanuel has also created a legal protection fund.

- Arguments on both sides -

Advocates say sanctuary cities offer needed protection to millions of undocumented migrants who work and pay taxes in the United States and who could be targeted and deported.

They also insist that the sanctuary policy encourages undocumented immigrants to report crimes and cooperate with police in investigations -- knowing that to do so will not call unwanted attention to their legal status.

Under the US Constitution, state and local governments have every right to refuse to help enforce federal law, said Michael Kagan, who heads the Immigration Clinic at the University of Nevada.

Detractors of sanctuary cities say these policies run counter to federal law and that such cities end up harboring criminals and endangering the public.

The debate on sanctuary policies intensified after the 2015 killing of Kathryn Steinle in San Francisco by a Mexican national with a criminal record who had been deported several times.