The State Department extended its closures of some US missions around the Arab world until Saturday amid security fears over an Al Qaeda threat that prompted more than 20 embassies and consulates to shut down this weekend.
A small number of additional posts will also be closed, while others would reopen Monday.
According to Reuters, posts in Abu Dhabi, Amman, Cairo, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Sanaa, Tripoli, Antanarivo, Bujumbura, Djibouti, Khartoum, Kigali and Port Louis will be closed through Saturday.
US diplomatic posts in Kabul, Baghdad, Algiers, Dhaka, Nouakchott, Herat, Mazar el Sharif, Basrah and Erbil will open on Monday, the State Department said.
Security was particularly strict Sunday in the Yemeni capital of Sanaa, where Britain, France and Germany all closed their embassies too following the US warning that lawmakers in Washington said involved Al Qaeda's Yemen and Saudi Arabia branch.
But the US alert spread across most Arab capitals and extended beyond the Arab world to Afghanistan and Bangladesh, prompting embassies to close on Sunday, the first day of the working week in many Islamic countries.
In Sanaa, special forces with armored personnel carriers were stationed outside the US Embassy and the missions of Britain, France and Germany, an Agence France-Presse correspondent reported.
Police and army checkpoints were set up on all the Yemeni capital's main throughfares.
Residents said they heard the sound of a drone flying over, which could only be American as Washington is the sole power to operate the unmanned aircraft in the region.
In Jordan, authorities beefed up security around the closed US mission.
"Authorities have conducted a sweep for explosives at all US diplomatic locations and beefed up security measures around the US Embassy," a Jordanian security official told AFP.
US National Security Adviser Susan Rice chaired White House talks to review Washington's response to the threat it revealed on Friday of major attack by Al Qaeda during the month of August.
Also attending were Secretary of State John Kerry, Defence Secretary Chuck Hagel and Homeland Security Secretary Janet Napolitano as well as the heads of the CIA, the FBI and the National Security Agency, the White House said.
President Barack Obama did not attend but was briefed afterward.
"Early this week, the president instructed his national security team to take all appropriate steps to protect the American people in light of a potential threat occurring in or emanating from the Arabian Peninsula," a White House statement said.
On Friday, Washington issued a worldwide travel warning, citing unspecified plans by Al Qaeda to strike US interests in the Middle East or North Africa in August.
Separately, Interpol issued a global security alert after hundreds of militants escaped jail. The worldwide police agency said it suspected Al Qaeda was involved in some of the jailbreaks across nine countries, notably Iraq, Libya and Pakistan.
It said the breakouts had "led to the escape of hundreds of terrorists and other criminals" in the past month alone.
Interpol noted that this week marks the 15th anniversary of Al-Qaeda's bombing of the US embassies in Nairobi and Dar es Salaam, which killed more than 200 people and wounded thousands.
August also marks the anniversaries of attacks in India, Russia and Indonesia.
Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told ABC News that the threats were directed at Western interests, and were "more specific" than previous threats.
While an exact target was unknown, "the intent seems clear. The intent is to attack Western, not just US, interests," Dempsey said.
The State Department issued a worldwide travel alert to US citizens, warning of the "potential for terrorists to attack public transportation systems and other tourist infrastructure."
The UK Foreign Office continued to urge Britons to leave Yemen. "Terrorists continue to threaten further attacks," it warned.
Al Qaeda rages over Egyptian leader's ouster
Hours after the US alert was issued, an audio recording was posted on militant Islamist forums in which Al Qaeda chief Ayman al-Zawahiri accused the United States of "plotting" with Egypt's military, secularists and Christians to overthrow Islamist President Mohamed Morsi.
In his first public comment on the July 3 military coup, the Egyptian-born Zawahiri said: "Crusaders and secularists and the Americanized army have converged ... with Gulf money and American plotting to topple Mohamed Morsi's government."
The United States has been especially cautious about security since an attack on its consulate in Libya's second city Benghazi on Sept. 11 last year.
The assault, blamed on Islamist militants, killed four Americans, including Ambassador Chris Stevens.
Washington considers Al Qaeda in the Arabian Peninsula to be the jihadist network's most active and dangerous branch.
The US government has waged an intensifying drone war against the group's militants in Yemen that executed three deadly strikes in the five days to Aug. 1.
Obama hosted Yemeni President Abdrabuh Mansur Hadi for White House talks hours after the latest strike in which they discussed joint efforts against the jihadists.