MANILA, Philippines » Security was increased at American embassies and consulates around the world on Thursday following an attack that killed the U.S. ambassador in Libya, while the U.S. urged its citizens abroad to be vigilant.

Guards and police special forces were seen carrying assault rifles outside the U.S. Embassy in the Philippine capital, while embassy guards gestured to a photographer to stop taking pictures.

President Barack Obama ordered increased security at American missions around the world after Ambassador Chris Stevens and three other Americans were killed Tuesday in an attack on the U.S. Consulate in Benghazi, Libya. The attack was presumed to have been triggered by a provocative American film that depicts the Islamic prophet Muhammad in disrespectful ways, but U.S. officials are investigating whether it was a terrorist strike planned to mark the 11th anniversary of the attacks of Sept. 11, 2001.

Chanting "death to America," hundreds of protesters angered by the film stormed the U.S. Embassy compound in Yemen’s capital and burned the American flag on Thursday.

The Yemeni Embassy in Washington condemned the attack and vowed to ensure the safety of foreign diplomats and to step up security measures around their missions in the country.

Indonesia’s government has condemned the anti-Islam film, "Innocence of Muslims," whose trailer has gone viral on YouTube. But there has been no public reaction so far in the world’s most populous Muslim nation, even though it is prone to large protests. Officials called on Indonesians to stay calm ahead of Friday prayers, when demonstrations often take place.

The U.S. Embassy in the Indonesian capital, Jakarta, issued a security message to American citizens advising them to pay close attention to their surroundings and to avoid large crowds that might turn violent.

"These events are a reminder to all of us that the security situation in any location can change rapidly and in unexpected ways," it said.

Indonesia’s government has asked Google, which owns YouTube, to help block online access to "Innocence of Muslims," said Gatot Dewabroto, a Communication Ministry spokesman. It was available Thursday morning, but could not be viewed by afternoon.

"The movie has hurt Muslims all over the world deeply. They deliberately wanted to make Muslims angry," said Amidan Shaberah, a prominent cleric at the influential Indonesian Ulema Council. "We urge Indonesian Muslims to calm down because the majesty and greatness of God and the Prophet Muhammad will not be diminished by these insults."

In Malaysia, a U.S. Embassy official said it was not the embassy’s policy to comment on its security measures. But the embassy said in an advisory on its website that "based on recent events in Cairo and Benghazi, there is the possibility of demonstrations taking place in Kuala Lumpur." The attack in Libya was preceded by violent protests in the Egyptian capital.

The embassy said it had no information about any planned demonstrations but noted that in the past, such gatherings could occur near the embassy on Fridays. It advised U.S. citizens in Malaysia to "exercise general caution, be aware of their surroundings, and avoid large crowds or gatherings."

In the Philippines, diplomats had asked for additional police personnel and patrols for the seaside compound that houses the U.S. Embassy, as well as a nearby residential complex and the consulate in the central city of Cebu, ahead of the anniversary of the Sept. 11, 2001, attacks in the U.S. Officials said the heightened security would be maintained indefinitely following the attack in Libya.

Traffic was busy as usual Thursday on a Manila boulevard in front of the embassy’s main entrance. A police pickup truck with a machine gun mounted on the back was parked under a tree, and Philippine coast guard vessels patrolled Manila Bay around the embassy. Police patrols were also intensified in other U.S. facilities, including the American cemetery in the Philippine capital.

"President Obama yesterday directed an increase in security at diplomatic posts around the world, and this includes our embassy here in Manila," said embassy spokeswoman Tina Malone.

Americans flags were lowered at half-staff as U.S. Ambassador Harry K. Thomas Jr. mourned the loss of the American diplomats in Libya.

"We will do our best to honor and carry forward their memory and their service," he said in a statement.

In Nigeria, a nation of more than 160 million people largely split between a Muslim north and a Christian south, the U.S. Embassy issued a warning saying that "extremists may attempt to target U.S citizens and other Westerners." Previous violence by a radical Islamist sect known as Boko Haram saw the embassy ban travel for its staff to the north for much of this year. The sect is blamed for killing more than 670 people this year alone, according to an Associated Press count.

However, Nigeria has remained largely quiet, with only one small protest over the video happening in the central Nigerian city of Jos, according to local media reports. Still, Nigeria’s federal police force has received orders to provide additional security to foreign embassies.

Associated Press writers Sean Yoong in Kuala Lumpur, Malaysia, Niniek Karmini in Jakarta, Indonesia, and Jon Gambrell in Lagos, Nigeria, contributed to this report.