President Barack Obama has condemned the killing of the US ambassador to Libya after an armed mob stormed the American consulate in Benghazi.
Chris Stevens, 52, and three other American officials died as gunmen protesting over a film they say insults the Prophet Mohammed fired rocket-propelled grenades and set fire to the US compound.
The Libyan doctor who treated Mr Stevens said he died of "severe asphyxia", apparently from smoke inhalation, and tried for 90 minutes to revive him.
Mr Obama said he "strongly condemned" the "outrageous and shocking" attack. "Make no mistake - justice will be done," he added.
Libya's deputy prime minister Mustafa Abu Shagur also condemned the "cowardly" attack in a Twitter message. Speaking in Cairo, British Foreign Secretary William Hague described the assault as "brutal and senseless".
In Afghanistan, Taliban leaders called on Afghans to "take revenge" on American soldiers for the US-made, anti-Muslim film.
Gunmen attacked the compound on Tuesday evening, clashing with Libyan security forces before the latter withdrew as they came under heavy fire.
Reporters at the scene said they could see looters raiding the building, walking off with desks, chairs and washing machines.
It followed an attack in the Egyptian capital Cairo, when protesters climbed the walls of the US embassy, tore down an American flag and burned it.
The protests were sparked by outrage over a video being promoted on YouTube by extreme anti-Muslim groups in the US.
Demonstrators say the \$5m film, called The Innocence Of Muslims, insults the Prophet Mohammed. It apparently depicts him as a fraud, showing him having sex and calling for massacres.
Sky's Defence and Security Editor Sam Kiley said the attack was a "violent and systematic" reaction to the film which he described as an "amateurish production".
"I have watched 15 minutes of it. It is contemptible, idiotic and very crude, and deliberately rude about the Prophet Mohammed. It was deliberately intended to inflame exactly the sort of reaction that has come now," he said.
"The tragedy is of course is that it's been successful. There are groups out in the Muslim world that rather than ignore this with the contempt that it is due, in my view, have been inflamed by it, or rumours of it."
Sam Bacile, the man behind the film, went into hiding on Tuesday. The 56-year-old property developer, who identifies himself as an Israeli Jew, said he produced, directed, and wrote the two-hour film.
Speaking by phone to the Associated Press from an undisclosed location, he remained defiant, saying Islam is a "cancer". He said he hoped his film would provoke reaction and expose what he described as Islam's flaws to the world.
Libyan officials said it remained unclear whether the two protests in Benghazi and Cairo had been co-ordinated.
Benghazi was the launch-pad of last year's revolution which overthrew Colonel Gaddafi and is a seat of Islamist politics in Libya.