Authorities arrested 596 illegal immigrants with prior criminal convictions in a three-day sweep across the Southeastern United States this week, the nation's top immigration enforcement officer said Friday.

Most of them will be deported, though 22 may be prosecuted for illegally entering the U.S. after previously having been sent home, said John Morton, head of the Immigration and Customs Enforcement agency, or ICE. He said the operation underscored the Obama administration's emphasis on deporting immigrants who break the law.

"These are not the kind of people that we want walking our streets or living in our communities," Morton told reporters.

The sweep also pointed to holes in the U.S. immigration enforcement system. All but two dozen of the criminals, some previously convicted of murder, sex crimes, assault and fraud, had been released from jail or prison without being turned over to immigration authorities for removal, as the law provides, Morton acknowledged.

By some estimates, there are as many as a million such criminal illegal immigrants in the U.S., he said.

"There are a lot of people who don't come to our attention and custody," said Morton, a longtime federal prosecutor and onetime Peace Corps volunteer.

A new program known as Secure Communities, which seeks to identify illegal immigrants in jails and prisons using fingerprints and other tools, is designed to tackle that problem, but will take time to fully implement, he said.

Most of this week's arrests occurred in Florida and in Puerto Rico, ICE said in a statement, with many others in Atlanta. Those arrested came from 60 different countries in Latin America, Asia, Europe, Eastern Europe, the Middle East and Africa, ICE said.

The sweep reflected a somewhat delicate approach the Obama administration has been taking to immigration enforcement as it mulls over its response to a controversial Arizona law targeting illegal immigrants and a new push in Congress for an immigration overhaul bill.

Latino groups and immigrant rights activists complain that authorities are continuing to deport workers and students who are no threat to the U.S. In response, Obama officials say they are focusing their efforts on people convicted of crimes, but acknowledge that they continue to remove noncriminals.

Meanwhile, the administration is also facing criticism from Congress that it is not aggressively enforcing immigration laws. In response, officials say they are removing a record number of immigrants, about 400,000 per year. A majority of those have not been convicted of crimes.

Democratic lawmakers, including Sen. Richard J. Durbin of Illinois and Sen. Dianne Feinstein of California, this week unveiled a comprehensive immigration package that provides a path to legalization for the estimated 11 million illegal immigrants currently living in the U.S. The bill also calls for tough new enforcement measures.

However, chances for passage before the November election are considered slim.