President Ali Abdullah Saleh, whose country joined the U.S.-led war against terrorism after the September 11 attacks, said Yemen had received warnings last week of an impending attack but did not know where or when the militants would strike.
He offered a $75,500 reward for information leading to the capture of militants linked to the attack at the Queen of Sheba Temple in the volatile eastern province of Marib.
"The security apparatus had information about four days ago but did not know exactly when or where this operation would be carried out," Saleh told a news conference.
"Security measures were taken around sensitive sites such as oil installations but we did not think of this temple."
Security sources told Reuters that al Qaeda issued a statement last week demanding the release of some of its members jailed in Yemen, which has been battling Islamist militants for years, and threatening to take unspecified action.
The fortified U.S. embassy in Sanaa cancelled all travel outside the capital and warned its citizens against travel to Marib, urging them to be vigilant throughout the country.
Yemen, viewed in the West as a haven for Islamist militants including al Qaeda, has seen several spectacular bombings.
Witnesses in Marib said body parts were strewn around the charred vehicles used by the Spaniards. One resident said the blast was strong and heard for miles around.
