Both now face jail terms of up to 14 years, according to US authorities.
Muhammad Bilal and Ahmad Bilal pleaded guilty in court in Portland, Oregon on Thursday and have agreed to cooperate with US criminal and military investigations, Attorney General John Ashcroft said in a statement.
The pair were among six arrested while training to fight for the Taliban government in the northwestern US state last October in a move that Ashcroft hailed as a "defining day" in America's War on Terror campaign.
Plea bargain confession
After plea bargaining, the two confessed to taking part in "martial arts training and firearms practice in order to prepare themselves to fight a violent jihad in Afghanistan or some other location at a point in the future," said the statement.
After 11 September 2001, the pair took shooting practice at a gravel pit in Washington state along with co-defendant Patrice Lumumba Ford and joined a group who planned to travel from Portland to Afghanistan to help fight US armed forces in Afghanistan.
The statement added that the Bilals admit that on 20 October, 2001, while US military forces were fighting al-Qaida and the Taliban in Afghanistan, co-defendant Mahir Hawash took them to Portland airport from where they flew to Hong Kong.
From Hong Kong, the pair traveled to Kashgar in western China were they tried, unsuccessfully, to gain entrance into Pakistan.
The Justice Department said that remaining charges against the two will be dismissed at the time of sentencing, which will be scheduled after the trial of the remaining defendants in January 2004.
Muhammad Bilal faces a probable sentence of 8-14 years at his sentencing. Ahmad Bilal faces a probable sentence of 10-14 years at sentencing.
Hawash, who was arrested separately from the other six defendants, entered a guilty plea in August to a charge of conspiracy to help the Taliban.