The warning, posted on the US Embassy website, specifically highlights the dangers posed by crowded areas in the Qassim region, which foreigners are known to frequent.

"We have received credible information that an unidentified extremist(s) in Saudi Arabia may be planning to attack Westerners working and living in Qassim, Saudi Arabia," the embassy said in the website statement. There are increased indications that terrorists are planning attacks in the Kingdom, said the alert, on which Saudi officials have refused to comment Saturday.

Maj. Gen. Mansour Al-Turki, a spokesman of the Ministry of Interior, said he could not immediately comment on the new alert issued by the US Embassy. The embassy, however, said that the Westerners face possible attack from unidentified extremists in the central province of Qassim.

"The timing and method of potential attacks are currently unknown," said the statement, urging US citizens to "exercise prudence and enhanced security awareness at all times."

Westerners in other parts of the Kingdom were, however, unfazed by this warning. “Life is normal here,” said one American in Jeddah.

Saudi Arabia arrested 113 mainly Saudi and Yemeni Al-Qaeda-linked militants in March.

Meanwhile, Al-Qaeda's Yemen-based arm said it was behind an attack that killed at least six soldiers in an oil province last month, and threatened more strikes on government targets.

The attack in the southern Shabwa province on July 25 was among five raids on state targets since June that have been blamed on the resurgent militant group.

Officials have said Al-Qaeda may have been also behind an attack that killed three soldiers on Thursday.

"Anyone who stands with (Yemeni President) Ali Saleh and his government, and with the Crusader (Western) campaign is against our Muslim people is our enemy and a legitimate target for us," Al-Qaeda in the Arabian Peninsula said in a statement posted on a website on Saturday.

The group previously focused its high-impact strikes on foreign targets but has started to aim at the state in response to enhanced US-Yemeni cooperation in a crackdown that has included airstrikes and raids.

— With input from agencies