US authorities arrested 2,900 illegal immigrants with criminal records during a seven-day nationwide sweep, the biggest of its kind, immigration officials said Wednesday.

The Immigration and Customs Enforcement agency said the operation, which it dubbed "Cross Check," "led to the arrest of more than 2,900 convicted criminal aliens."

The operation was carried out over a one-week period in all 50 US states and overseas territories, as part of the US administration's strategy to focus on people with criminal records who are in the country illegally.

"The estimates vary but (there are) somewhere around a million people with criminal convictions who are subject to removal under the law," said ICE director John Morton.

He said the number of illegal immigrants in the country -- about 11 million according to official estimates -- has always been more than the agency can expel.

"From our prospective, the best place to start is with criminal offenders," he said at a new conference.

Of those detained in the latest operation, 1,282 had multiple convictions to their name, and more than 1,600 had served sentences for crimes like armed robbery, attempted murder, kidnapping or drug trafficking, ICE said.

It said 681 of those detained had been expelled from the United States after being convicted of crimes but returned to the country illegally, it said.

The United States has expelled 350,000 illegal immigrants a year on average over the past three years -- a record high -- and about half of them had criminal records.

The agency has carried out operations like "Cross Check" before, but this was the first time it has done so simultaneously throughout the country.

About 1,900 agents took part in the sweep, it said.

Most of the undocumented immigrants living in the United States are Hispanics.