"It definitely is a problem for us if local law enforcement are encouraged not to cooperate with ICE or not to work with ICE, even when we're talking about known gang members who may have committed very, very serious crimes," said Julie L. Myers at a press conference to announce the results of a national gang enforcement operation.

Myers said progress had been made in some cities where local law enforcement officers had been killed by illegal aliens, but there were still complications in other unspecified cities.

"We work to see, can we agree that we don't want transnational, alien gang members, and if so, can we partner specifically on that, and we've had good luck," Myers said.

A federal law enforcement source, who spoke on condition of anonymity, confirmed ICE agents in California face the greatest problems in San Francisco, where a citywide sanctuary policy bars immigration officers from reviewing arrest records or training local jailers on federal immigration law.

In Los Angeles, the source said, ICE agents have better access to inmates and local jailers have been trained to identify arrestees who may be living in the U.S. illegally.

Myers and ICE announced Wednesday more than 1,700 criminal gang members have been arrested nationally as part of, "Operation Community Shield," an ongoing effort to catch violent criminals.

293 of the arrests were made in Southern California, ICE officials said.