By RACHEL L. SWARNS
 
WASHINGTON, Aug. 10 — Citing concerns about terrorists crossing the nation's land borders, the Department of Homeland Security announced today that it planned to give border patrol agents sweeping new powers to deport illegal aliens from the frontiers abutting Mexico and Canada without providing the aliens the opportunity to make their case before an immigration judge.

The move, which will take effect this month, represents a broad expansion of the authority of the thousands of law enforcement agents who currently patrol the nation's borders. Until now, border patrol agents typically delivered undocumented immigrants to the custody of the immigration courts, where judges determined whether they should be deported or remain in the United States.

Homeland Security officials described the immigration courts — which hear pleas for asylum and other appeals to remain in the country — as sluggish and cumbersome, saying illegal immigrants often wait more than a year before being deported, straining the capacity of detention centers and draining critical resources. Under the new system, immigrants will typically be deported within eight days of their apprehension, officials said.

Immigration legislation passed in 1996 allows the immigration service to deport certain groups of illegal aliens without judicial oversight, but until now the agency only permitted officials at the nation's airports and seaports to do so. The new rule will apply to illegal aliens caught within 100 miles of the Mexican and Canadian borders who have spent 14 days or less within the United States. The border agents will focus on deporting third-country nationals, rather than Mexicans or Canadians, and they are expected to begin exercising their new powers on Aug. 24 in Tucson and Laredo, Tex.

"There is a concern that as we tighten the security of our ports of entry through our biometric checks that there will be more opportunity or more effort made by terrorists to enter our country through our vast land borders," Asa Hutchinson, the undersecretary for border security at the Department of Homeland Security, said at a news conference.

"We recognize that we have to secure those and that's the president's first principle of immigration reform," Mr. Hutchinson said. "America must secure its borders and this is a part of that effort."

The decision was hailed by officials who have long complained that the nation's porous borders represent a serious threat to national security. But it prompted a flurry of criticism from advocates for immigrants who warned that the new system lacked adequate safeguards to ensure that people fleeing persecution, American citizens lacking paperwork or other travelers with legitimate grounds would not be improperly deported.

Mr. Hutchinson said that border agents would be trained on asylum law and that immigrants who demonstrated a credible fear of persecution would be sent to see immigrant judges, not returned to hostile governments. "That right," he said of the right to apply for asylum, "is very important."

But Homeland Security officials provided little details about the training, and advocates said that they feared that mistakes would be made when border agents decide who will be deported and who will not, often in the vast, inhospitable plains of the southern deserts.