Britain's Foreign Office said Friday it believed that "terrorists may be in the final phases of planning attacks" in Saudi Arabia.

The Foreign Office gave no details about its information, but said its warning against all but essential travel in Saudi Arabia remained in place.

The Foreign Office said it had updated its travel advice, saying, "We advise British nationals against all but essential travel to Saudi Arabia. We believe that terrorists may be in the final phases of planning attacks."

A statement posted on the department's Web site advised travelers to make sure they had confidence in their security arrangements and that visitors to military buildings should take special care.

A U.S. counterterrorism official, speaking on condition of anonymity, said American authorities were unaware of any recent intelligence that would lead to new alerts in Saudi Arabia.

Instead, U.S. officials have received a steady stream of information in recent months suggesting Al Qaeda operatives in the kingdom were close to mounting an attack.

The FBI urged extra vigilance for possible terror attacks and violence against Muslims during the upcoming Islamic holy month of Ramadan (search).

In its weekly bulletin to 18,000 state and local law enforcement agencies, the FBI says it has no credible information that an attack is planned by Al Qaeda or any other terror group during the period of fasting and reflection that begins next week.

But attacks overseas have been timed in the past to coincide with symbolic dates, the FBI says, adding that "the possibility of such an attack in the United States cannot be discounted."

On Thursday, Australia warned that another terrorist attack in Saudi Arabia "may be in the final stages of planning" and urged its citizens to avoid going there. The Australian government did not elaborate but it also authorized families of Australian Embassy staff to leave Riyadh (search), the Saudi capital.

On May 12, car bomb attacks on three Western housing compounds in Riyadh killed 26 people and the nine assailants. Saudi Arabia linked Usama bin Laden's Al Qaeda terror network to the attacks and then cracked down on Islamic militants in highly publicized raids.

The official Saudi Press Agency reported Thursday that the country had arrested 600 people suspected of having Al Qaeda links.

The Saudi ambassador to London, Prince Turki al-Faisal (search), said in London on Wednesday that more than 400 of those suspects remained in custody, the agency reported.

A U.S. official said Thursday that Saudi Arabia has shared more information with American law enforcement authorities since May. The countries also set up a joint counterterrorism operations center, the official said on condition of anonymity.

The kingdom faced increased scrutiny following the disclosure that 15 of the 19 suicide bombers involved in the Sept. 11 attacks on the United States were Saudi nationals, as is bin Laden.