An American was one of two people killed in a bomb attack that has highlighted tensions in the region ahead of possible U.S. military action against Islamic militants.
At least four other foreigners -- a second American, a Briton and two Filipinos -- were injured, the official Saudi Press Agency (SPA) and hospital officials said.
But as an investigation began on Sunday in the eastern city of Khobar, where five years ago 19 U.S. servicemen died in an earlier bombing, there was no immediate evidence of a link to the U.S. military build-up in the region against Muslim groups it blames for last month's attacks on the United States.
A source close to the Saudi police said the second, as yet unidentified, person who died might have been carrying the bomb and was probably a Pakistani, Indian or Afghan. One local media report quoted witnesses talking of a "suicide" attack.
Osama bin Laden, the Saudi-born militant whom Washington blames for the September 11 attacks and who is now believed to be in Afghanistan, is a violent critic of the Saudi royal family and its close ties to the United States.
But U.S. officials saw no immediate link to the suicide attacks on New York and Washington: "We believe this is an isolated incident," one official told Reuters in Washington.
A string of bombings that rocked the conservative Muslim kingdom last year were linked by Saudi newspapers to a lucrative trade in illegal alcohol among foreigners.
The blast around 8 p.m. local time on Saturday struck a shop in a commercial district popular especially with Asian workers.
SPA named the dead American as Michael Gerald Martin. The U.S. embassy declined to identify the Americans until their families had been notified. It said neither was in the military.
In London, the Foreign Office confirmed that one Briton was slightly injured.
Investigators were trying to identify the second body, which had been blown to pieces, the source close to the police said.
"It seems the person was an Asian, most likely a Pakistani, Indian or Afghan," the source said.
"It is possible this person was holding the explosive."
A Saudi journalist told Qatar's al-Jazeera television that witnesses described the explosion as "a suicide attack and the perpetrator was... most possibly a Pakistani or an Afghan."
The U.S. embassy, in a recorded message, advised Americans to check their cars, keep a low profile and take precautions.
"To date, no individual or group has claimed responsibility for the bombing," callers to the Riyadh embassy were told.
"Latest reports indicate that two individuals were killed and five injured in the blast. It is believed that one of the deceased and one of the injured were Amercans."
The British embassy said it would advise its 30,000 or so nationals to tighten security as a result of the blast.
"The perpetrators of this cowardly, heinous crime will fall in the hands of justice," SPA quoted Saudi Arabia's Crown Prince Abdullah as saying.
In June, the United States charged 14 suspected Muslim militants, mostly Saudis, for the 1996 Khobar Towers bombing.
The Saudi government said last month some suspects in the blast had disappeared and might have fled to Europe.
A nurse at Khobar's King Fahd Hospital said the injured were expected to stay under observation for three to four days.
Khobar, a city of malls that has seen several bomb attacks against Westerners in recent years, was in shock on Sunday.
Saudi television showed the interior of the shop, which sells watches and electronics, littered with twisted metal and shattered glass. Blood lay in pools inside and outside the store and also spattered a car parked nearby.
"We were sitting with friends and we heard a big bang," one Saudi man said. "We thought it was an electrical short-circuit. Then I saw people walking around with blood pouring from them."
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