A message posted on an al-Qaida web site has claimed responsibility for Friday's attempt to blow up the world's largest oil refinery, in eastern Saudi Arabia.
The al-Qaida message - which has not been authenticated - calls the attackers "holy warriors" and says the attack is part of a series of operations aimed at expelling "infidels" from the Arabian peninsula.
It calls the attack a success, and says bombers penetrated the security perimeter of the oil and gas refinery.
Saudi authorities said the attack was foiled when guards opened fire on two attack vehicles. Two security officers and two attackers were reported killed.
The U.S. Ambassador in Riyadh, James C. Oberwetter, praised the Saudi security services Friday for foiling the attack. He said the kingdom deserved "considerable credit" for enhanced security at its oil facilities.
The Saudi government says the explosion did not affect operations at the plant. Nevertheless, oil prices jumped more than two dollars a barrel on Friday.
This is the first time that terrorists have attacked a Saudi oil facility.
The Abqaiq plant handles two thirds of the Kingdom's oil output and is run by Saudi Aramco, which took over Saudi oil production in the 1970s.
Several thousand foreigners work in the Saudi oil sector.
Some information for this report provided by Reuters and AFP.