Lieberman, a former Democratic vice-presidential candidate, said it was too early to definitively state the motives of Nidal Hasan, who is alleged to have killed 13 and injured 30 on a murderous rampage last week, but said clues pointed to terrorism.

"There are very, very strong warning signs here that Dr Hasan had become an Islamist extremist and, therefore, that this was a terrorist act," he told Fox News.

"It's clear that he was, one, under personal stress and, two -- if the reports that we're receiving of various statements he made, acts he took are valid -- he had turned to Islamist extremism," he said.

"If that is true, the murder of these 13 people was a terrorist act."

Lieberman chairs a senate panel that is to probe whether the army missed warning signs that Hasan, a army psychiatrist, may have been preparing an attack.

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