Department of Homeland Security (search) pilot program taking place in Denver and Atlanta is trying to make it harder for aliens to skip out on court appearances and disappear.

In the past, foreigners appealing immigration decisions were not locked up, but many disappeared and never showed up for their court dates. Under Operation Compliance (search), they would be jailed until the appeals process is over.

But immigrant advocates say that treating aliens like criminals is a mistake.

"This is who we're talking about: People who have good cases, the kind of people we want to welcome into this country, the people that make America strong and vital. These are the people we're talking about putting into jail," said Denver immigration lawyer Laura Lichter.

Corina Almeida of the U.S. Immigration and Customs Enforcement (search) said that the program could be an important tool in jailing criminal aliens.

"These are aliens who have been convicted of murder, rape, drug trafficking, driving while intoxicated — some pretty serious crimes," she said.

The pilot program will last 120 days. If successful, it will be expanded nationwide.

Click here to watch a report by Fox News' Carol McKinley.