Concerns about Yemen
U.S. officials said that based on intelligence, they were particularly concerned about the U.S. Embassy in Yemen between Saturday and Tuesday. President Barack Obama, amid regular updates on the situation, has directed officials to take all appropriate steps to protect Americans.
Hundreds of additional security forces were deployed, and roads leading to the embassy were closed. Checkpoints were set up at a distance from the embassy, and trucks weren't allowed to pass anywhere near the main embassy road.
Yemen's special forces, the most elite, were seen in small numbers near the embassy as well.
The UK Foreign Office has also warned its citizens against travel to Yemen, and it urged British nationals to leave as soon as possible.
"If you don't leave the country now while commercial carriers are still flying, it is extremely unlikely that the British government will be able to evacuate you or provide consular assistance," it said.
In Sanaa, some said concerns about attacks were overblown.
Wagdi Al-Absi, a university professor based there, told CNN that the threats and warnings are exaggerated.
"Al Qaeda was a real threat and very powerful in Yemen two years ago. Still, embassies did not close," he said. "Today, when al Qaeda is handicapped, the West considers them a real threat. It's the total opposite.
"Streets in Yemen are normal, and security forces are always on the watch. That is why al Qaeda attacks in the capital are not common."
Time of attack unknown
The expected time of an attack is unknown, which explains why a U.S. travel alert extends through August.
"Terrorists may elect to use a variety of means and weapons and target both official and private interests," the alert states. "U.S. citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure."
U.S. Rep. Peter King of New York, a member of the House Intelligence Committee, called the information "the most specific I've seen."
While the principal attention is on the Arabian Peninsula, he stressed to CNN's Wolf Blitzer that "we can't rule anything out."
"We are focused on the Middle East, but it's a potential series of attacks that really could be almost anyplace," King said.
U.S. forces on alert
Select U.S. military forces in the Middle East were put on a higher state of alert.
Defense Secretary Chuck Hagel held a series of high-level meetings at the Pentagon "to take a look at what forces we have in the region" that can be used if there is an attack, a U.S. official told CNN.
These forces are at "quite a high state of readiness achieved only a handful of times in the last year," the official said. He declined to be specific, but two other officials have told CNN that U.S. Navy amphibious ships in the Red Sea last week moved closer to Yemen.
There are also combat-equipped Marines in southern Spain and southern Italy that could begin moving in as little as one hour from getting orders.
The U.S. military has taken similar action in recent months, especially at times of unrest in Yemen and Egypt. These forces in the Red Sea, Spain and Italy have been regularly deployed to these areas since last year's attack in Benghazi, Libya, when the U.S. military did not have forces nearby.
Christopher Hill, a former U.S. ambassador to Iraq, told CNN he had never seen embassy closures ordered across such a broad area.
"There have been incidents where they've closed down a number of embassies in the Middle East because the information is not specific enough to say that 'embassy X' got to be closed as opposed to other embassies," said Hill, who joined the U.S. Foreign Service in 1977.