Saudi Arabia issued the names and photos of its 26 most wanted terrorist suspects and increased protection around Western housing compounds in the capital Saturday as the United States upgraded its security warning, restricting its diplomats' movements.

The new U.S. warning ordered embassy and consulate staff not to leave the heavily guarded diplomatic quarters in Riyadh (search) and other cities, except for essential duties.

The steps came days after the United States and Britain warned that militants were scoping out Western residence complexes for a possible new terrorist attack following two homicide bombings on compounds this year that killed a total of 52 people.

Separately, an American and a Briton have been detained by Saudi authorities in Jiddah (search), the U.S. and British embassies said Saturday, but embassy officials said they did not know the reason. A state-controlled newspaper identified the American as the brother of two people who have pleaded guilty in the United States to conspiring to help Al Qaeda (search).

A U.S. diplomat declined to say if the new restrictions on diplomats' movements were in response to a specific terrorist threat. Saudi security forces have strongly defended the Riyadh Diplomatic Quarter, an area of about five square miles west of the city, since suicide bombers attacked three housing compounds for foreigners in the capital on May 12.

The Saudi government on Saturday published the names and photographs of its 26 most wanted people, alleging they were "connected to the terrorist events in the kingdom over the past months." The list included one Yemeni citizen, two Moroccans and 23 Saudis.

In a release aired on state television and released on the official Saudi Press Agency, the Interior Ministry said Saturday it was offering a reward of $267,000 to information leading to the arrest of one of the 26 wanted. The reward would rise to $1.3 million for information leading to the arrest of more than one wanted person, and to $1.9 million for actions that foil an attempted terrorist attack.

At the Seder Village compound — specified by U.S. and British officials last week as a possible target — policemen with pistols were replaced by Special Forces troops with heavy machine-guns on Saturday. Additional roadblocks were installed.

At the Arabian Homes, another compound where expatriates tend to live in Riyadh, there were extra National Guardsmen in armored personnel carriers with their fingers on the trigger, said a resident, William Barilika, an American businessman from Stamford, Conn.

"I was puzzled when I came home from work ... I asked myself 'I wonder what's going on?'," said Barilika.

The Saudi government launched a widespread crackdown on Islamic militants and Al Qaeda cells on its soil following the May suicide bombings, which killed 35 people, including nine attackers. Hundreds of suspects have been arrested. On Nov. 8, a new suicide attack on a Riyadh housing compound killed 17 people. Both attacks have been blamed on Al Qaeda.

Meanwhile, a U.S. diplomat at the consulate in Jiddah confirmed that Saudi police arrested an American citizen but told The Associated Press, "we don't know why." British Embassy spokesman Barry Peach identified the British detainee as David Heaton, a convert to Islam.

"We have contacted his next of kin, and we are seeking reasons for his detention from the Saudi authorities," Peach said.

The U.S. diplomat, who spoke on condition of anonymity, refused to identify the American.

A state-controlled newspaper, Arab News, said the American was Abdullatif Ibrahim Bilal. It reported he is the brother of Ahmed and Muhammad Bilal, who are awaiting sentencing by a U.S. court after pleading guilty to charges of conspiring to help Al Qaeda and the Taliban and to violating the law on firearms. They face up to 14 years in prison.

Ibrahim Bilal, father of the three brothers, refused to comment when the AP called him. Arab News quoted him as saying his son had been detained, but he had no idea why.

While both men were detained in Jiddah, it was not clear if they were linked.

Hours before the Nov. 8 attacks, the U.S. Embassy in Riyadh and consulates in Jiddah and Dhahran closed, warning an attack could be imminent. The three missions reopened a week later.

The Australian and British embassies also upgraded their security warnings shortly before Nov. 8, saying they had received information of a specific threat.

Earlier this month, Britain repeated its warning on Saudi Arabia, telling its citizens it advised against "all but essential travel" to the kingdom.

"Following terrorist attacks in Riyadh in May and November, we continue to believe terrorists are planning further attacks in Saudi Arabia," the British Embassy said on Dec. 2.