A US National Guard soldier was sentenced to life in prison after being convicted of trying to aid the al-Qaeda network, authorities said today.

Specialist Ryan Anderson, who converted to Islam, was sentenced by a nine-officer jury in a court martial at the Fort Lewis Army base south of Seattle, in the northwestern state of Washington.

"He was sentenced to confinement for life with possibility of parole and a dishonorable discharge, with reduction to the rank of private," a military statement said.

The Army had decided before the court martial that it would not to seek the death penalty.

Anderson, who joined the National Guard in May 2002, was reported to have attempted to make contact with al-Qaeda through Internet chat rooms.

His activities were uncovered by Shannen Rossmiller, a municipal judge from Montana, who in her spare time sought to catch terrorists on the Internet.

Judge Rossmiller told prosecutors that while she was monitoring a website devoted to radical Muslims she happened upon an e-mail posting from one "Amid Abdul Rashid", who turned out to be Anderson.

The judge, masquerading as an extremist Muslim, began to correspond with Anderson. When she found out that he was a soldier, she contacted the Federal Bureau of Investigation.

Anderson was arrested in February at his apartment in Lynnwood, Washington, just before his unit was to deploy to Iraq.