U.S. authorities are giving thousands of foreign visitors from 18 predominantly Muslim countries a second chance to register with immigration officials.

Males older than 16 from the designated countries are required to register. These countries include Iran, Iraq and Syria as well as Afghanistan and North Korea. Most of those required to register are in the United States as students or business travelers, or are visiting relatives.

A notice in the Federal Register, the official publication of U.S. laws and regulations, said the Immigration and Naturalization Service is reopening registration for people who were required to report to the INS earlier this month in the multi-phase process. Authorities say some individuals were not aware of the registration requirements in the initial phases.

Immigration lawyer Pauline Schwartz, who was helping her Moroccan client register with INS authorities last week, agreed.

"I think a lot of people weren't sure if this applied to them or not. The law is a little bit confusing," she said.

U.S. immigration authorities began the registration process last year for visiting male citizens from countries considered sponsors of terrorism, or believed to harbor terrorists.

The initial deadlines in the registration effort resulted in hundreds of arrests of immigrants, who are accused of violating immigration laws. Immigrant rights and civil liberties critics say the program discriminates against Muslims, leads to arbitrary detentions and has not been well enough publicized.

Algerian student Hassen Cherifi, who registered during the initial period earlier this month, says he waited until what he thought would be the last day, to ensure he had all his paperwork in order.

"I had to wait for my documents to get ready," he said. "I wasn't going to show up with missing documents that they required."

Under the registration process, foreign visitors are required to come in to INS offices to be photographed, fingerprinted and interviewed under oath.

Different deadlines have been imposed for people from different countries. Citizens from Pakistan and Saudi Arabia, the largest group, so far, are required to register by February 21.

In addition, authorities are expanding the list of countries whose visiting male citizens are required to register, adding Bangladesh, Egypt, Indonesia, Jordan and Kuwait.