Obama Vows to 'Stand Fast' amid Spread of Anti-U.S. Violence

President Barack Obama has vowed to "stand fast" against spreading anti-U.S. violence raging in the Arab world, as he mourned four Americans slain in Libya after their remains were flown home.

In a heart-rending homecoming, four transfer caskets draped in American flags were slowly borne Friday from the belly of a giant C-17 transport plane by Marines in dress uniform, and set down in a hangar.

U.S. ambassador Chris Stevens and the three other Americans died Tuesday when a mob furious over an anti-Muslim Internet video made on U.S. soil torched the American consulate in Benghazi, part of a region-wide eruption of violence.

"Greater love hath no man than this, that he lay down his life for his friends," Obama said, quoting the Bible as he honored four "American patriots" for embodying national qualities of courage, hope and idealism.

"Their sacrifice will never be forgotten, we will bring to justice those who took them from us. We will stand fast against the violence on our diplomatic missions," Obama said at Andrews Air Force Base outside Washington.

As he spoke, a furious wave of anti-American violence ripped across the Middle East and North Africa, with a crowd invading the U.S. embassy compound in Tunis, and guards at the U.S. embassy in Khartoum firing warning shots at protesters.

Fresh violence erupted in Yemen and Cairo and demonstrations took place in Bangladesh, Indonesia, Malaysia, Pakistan, Iraq, Israel and the Gaza Strip, Morocco, Syria, Kuwait, Nigeria and Kenya.

Six people were killed and dozens injured in Friday's violence alone.

Obama said that the "awful" loss and terrible images may cause some to question the dangerous work of U.S. diplomats abroad, but argued that America must not abandon its global mission to spread dignity and freedom.

"Even as voices of suspicion and mistrust seek to divide countries and cultures from one another, the United States of America will never retreat from the world," he vowed.

Secretary of State Hillary Clinton told grieving relatives of Stevens, information officer Sean Smith and former Navy SEALs Tyrone Woods and Glen Doherty that the killings dishonored the spirit of the Arab Spring.

"The people of Egypt, Libya, Yemen and Tunisia did not trade the tyranny of a dictator for the tyranny of a mob," she said.

U.S. military and intelligence agencies have already launched a manhunt in Libya for the militants who staged the assault on the consulate in Benghazi.

"There's an intense focus" on finding the attackers, a U.S. defense official told Agence France Presse.

The violence erupted in response to excerpts of the "Innocence of Muslims" video mocking Islam that went viral on YouTube.

National Security Council spokesman Tommy Vietor told AFP the White House "reached out to YouTube to call the video to their attention and ask them to review whether it violates their terms of use."

On Wednesday, the Google-owned site said the film was within the firm's guidelines and would not be taken down, though access to it was temporarily restricted in Egypt and Libya.

Officials said Obama was being repeatedly briefed on the worsening security situation around U.S. posts in the Middle East and elsewhere, and was mounting a strong diplomatic push with key regional leaders to restore calm.

He has spoken personally to the leaders of Egypt, Libya and Yemen, and the White House said he sent a personal message to Turkish Prime Minister Recep Tayyip Erdogan asking him to speak out against the violence.