"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.
It was unclear whether the apparent plot targets that Arabian nation or one elsewhere -- which is why the travel alert applies so broadly, and why embassies from Bangladesh to Libya are being closed. Nor is the expected time of an attack known, which explains why the 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."
New York Rep. Peter King, 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.
22 embassies, consulates ordered closed
The State Department listed the 22 embassies and consulates that are closed Sunday, which is normally the start of the work week in the countries affected.
The 17 affected U.S. embassies are in Afghanistan, Algeria, Bahrain, Bangladesh, Djibouti, Egypt, Iraq, Jordan, Kuwait, Libya, Mauritania, Oman, Qatar, Saudi Arabia, the United Arab Emirates, Sudan and Yemen. The U.S. Embassy in Israel will be closed as normal Sunday.
Consulates in Iraq, Saudi Arabia and the United Arab Emirates are also being shut for the day. Embassies and consulates in the region typically close or operate with minimal staff on Fridays and Saturdays.
The shutdowns could extend beyond Sunday, a senior State Department official said.
Retired Gen. James Mattis -- who until earlier this year was head of U.S. Central Command, responsible for a 20-country area that includes the Middle East -- said the decision to close the embassies underscores the reality of the threat and the wisdom of U.S. policymakers.
U.S. embassies have been targeted before in places such as Yemen, Turkey and Tanzania, he pointed out. Moreover, al Qaeda in the Arabian Peninsula is one of the terrorist network's most active and most destructive branches.
"We have to remember that we're up against an enemy who kills indiscriminately -- whether it be women, children, diplomats -- and our embassies ... have been one of the targets," Mattis told CNN on Friday.
Referring to the move by U.S. officials, he said, "They are showing some proactive discretion here, making certain that we don't give the enemy an opportunity that we can deny them."
Meanwhile, Canada shuttered for one day its diplomatic mission in Bangladesh on Saturday.
Questions, concerns after Benghazi
House leaders have been briefed, Democratic leader Nancy Pelosi told reporters, adding that the travel alert and embassy closings provided "some understanding of the seriousness of the threat."
King, who has also heard such briefings, applauded the government's decision to close its diplomatic missions.
"I give them credit," the Republican said of the Obama administration. "I think the government is doing exactly the right thing here."
Such bipartisan agreement in Washington comes at a time when politicians are still scrutinizing the September 11, 2012, attack on the U.S. consular compound in Benghazi, Libya, which killed four Americans, including Ambassador Christopher Stevens.
Since then, Republicans have been pressing Obama's administration for answers, with some accusing officials of covering up what happened in Benghazi and not doing enough to track down the attackers.
Eight GOP lawmakers are asking that incoming FBI Director James Comey brief Congress within 30 days about the investigation. They say the administration's inquiry to date has been "simply unacceptable," according to a draft letter obtained by CNN.
Earlier this week, Vice President Joe Biden and senior State Department officials went to Congress to discuss embassy security.
Biden also briefed congressional leadership, key committee chairmen and ranking members about the latest threat concerns, a source who attended the meeting said.