A US airman who was on guard in Saudi Arabia when a fuel truck exploded killing 19 of his comrades, said today he felt they were being watched.

``I guess you would say yes, we had a feeling we were being watched.......ever since we got the threat,'' said Staff Sergeant Alfredo Guerrero, who was on the roof of the bombed apartment block in nearby Khobar on Tuesday night.

Guerrero was referring to a threat of retaliation made in May if four Saudis were executed for a previous bombing in which five Americans and two Indians were killed.

The four were publicly beheaded and the base was shut down for a few days.

Earlier today, servicemen at the apartment complex scrambled to safety after a caller threatened another explosion. It turned out to be a hoax.

Two previously unknown groups opposed to the presence of US forces in Saudi Arabia claimed responsibility for Tuesday's bomb. There was no firm indication of who had detonated it.

Guerrero, 29, from Modesto, California, told a news conference he was one of three on security patrol on the roof when the fuel truck pulled up.

They went down as far as the sixth floor to evacuate people. Three-and-a-half minutes later the truck exploded.

``The two men driving the truck got into a passenger vehicle and proceeded to drive off. I figured that there was something wrong with that,'' Guerrero said.

``They were either running or it was a very brisk walk. It was definitely obvious that they wanted to get away.''

Guerrero, who was cut under his right eye, said: ``It did not knock me down but it kind of spun me around a little bit.''

Senior Airman Chris Wagar of Washington state, who was on the roof with Guerrero, said: ``We really did not have time to be scared. Me, personally, I saw my life flash before my eyes.''

Lieutenant Colonel Dennis Aleson, a Methodist preacher, told reporters they were working non-stop to counsel people suffering from ``shock, anger, and the guilt of surviving''.-Reuter.