Republican lawmakers on Capitol Hill are taking aim at the leaders of four so-called sanctuary cities in the U.S. over their policies on the topics of immigration and crime.

The Democratic mayors of New York City, Boston, Chicago, and Denver are appearing Wednesday before the House Oversight and Government Reform Committee, fielding questions over why they aren't doing more to help detain and deport migrants.

RELATED STORY | New lawsuit against Trump administration over 'illegal targeting' of sanctuary cities

In his opening statement, committee Chairman James Comer blamed the Biden administration for the "border crisis," and questioned why the mayors are allegedly hindering President Donald Trump's crackdown on illegal immigration.

"The mayors here today each lead a so-called 'Sanctuary City'. And let's be clear: these policies only create sanctuaries for criminals," Congressman Comer said in his prepared remarks. "Sanctuary policies violate federal immigration law by protecting criminal aliens at the expense of the American people."

RELATED STORY | Four FEMA employees fired over 'egregious payments' for NYC migrant housing

New York City Mayor Eric Adams rejected those claims, stating that his "top priority" has been to bring crime down in his city. He also noted that New York City has seen three consecutive months of declines in "major crimes."

"To be clear: A 'sanctuary city' classification does not mean our city will ever be a safe haven for violent criminals," Adams said. "It also does not give New York City the authority to violate federal immigration laws."

"As mayor, I do not control who enters or remains in our country, but I do have to manage the population that is within our city," Adams added. "In order to carry out this function without having long-term negative ramifications, I must create an atmosphere that allows every law-abiding resident, documented or not, to access vital services without fear of being turned over to federal authorities."

RELATED STORY | As border crossings plummet, White House wants more money for apprehensions

The hearing comes as President Trump has touted his crackdown on illegal immigration. During his Tuesday night address before a joint session of Congress, he claimed "murderers, human traffickers and gang members" had entered the U.S. thanks to lax border policies under the Biden administration.

"We quickly achieved the lowest number of illegal border crossings ever recorded," he said of his administration's policies so far.

In an executive order signed on the day he took office, President Trump designated several cartels as foreign terror organizations, to facilitate the removal or imprisonment of their members in the U.S. The president has also asked Congress for more funding to expand deportation operations, promising to sign legislation as quickly as it's delivered to his desk.

"The cartels are waging war on America, and it's time for America to wage war on the cartels," he said.