The Bush administration urged U.N. investigators Tuesday to look into an allegation by a former Syrian vice president that President Bashir Assad had threatened former Lebanese President Rafik Hariri several months before Hariri was assassinated.

The allegation was made last week by former Vice President Abdul-Halim Khaddam, who left Syria several months ago.

Khaddam said Assad warned Hariri in August 2004 that he would "crush whoever attempts to overturn our decision" to extend the term of Lebanon's pro-Syrian president.

Hariri, a billionaire Lebanese nationalist who sought to assist his country wiggle free from Syrian control, was assassinated last February in Beirut. The blast killed 20 other people and helped accelerate U.N. demands that Syria release its grip on its smaller neighbor.

Referring to Khaddam's allegation, State Department spokesman Sean McCormack said, "I think his remarks warrant further investigation by Mr. (Detlev) Mehlis."

Mehlis already has implicated senior Syrian officials in Hariri's assassination. Syria has dismissed the charges as unfounded.

"I think that Mr. Haddam's remarks raise some deeply troubling issues as to what exactly was going on during the period in time in question," McCormack said.

"They raise serious questions about who in the Syrian government may have been involved in the assassination," he said.