SANAA — Demonstrators stormed the US embassy in Yemen on Thursday, leading to clashes in which four people were killed, while protesters stoned Washington's mission in Cairo as anger spread over a US-produced film mocking Islam.
A security official said 34 people were also wounded in the clashes "that lasted from morning until late in the evening," and that eight were in serious condition.
Witnesses said police fired live rounds tear gas to disperse hundreds of protesters while troops deployed on rooftops of buildings around the mission in Sanaa.
Earlier in the day police used water cannon and fired warning shots to expel protesters who breached the perimeter wall.
One demonstrator was shot dead outside the compound as police battled to prevent any new incursion, with three others killed in successive clashes.
The protests came as US and Libyan officials said they were probing a mob attack on the consulate in Benghazi that killed the ambassador and three other US officials on Tuesday, amid growing speculation it was the work of jihadist militants rather than just demonstrators.
As Libya announced it made arrests, US Attorney General Eric Holder told an Arab forum in Qatar, "the FBI has opened an investigation" into the deaths and the attack on the consulate.
Yemeni President Abdrabuh Mansur Hadi apologised to his US counterpart Barack Obama and the American people for the acts of a "mob" and ordered a probe.
"Those who are behind (the attack) are a mob that are not aware of the far-reaching plots of Zionist forces, especially those who made a film insulting the Prophet," said Hadi.
Some protesters said they saw three vehicles being torched by some of the demonstrators after they gained access to the compound through an unguarded security gate.
After being evicted from the complex on their first assault, protesters retreated about 100 metres (yards) from the gate, gathering near a checkpoint where they chanted anti-Jewish slogans.
Chanting "O, messenger of Allah... O, Mohammed," the protesters then launched a second bid to access the compound, prompting police to fire on the crowd, witnesses said, and clashes continued until late in the evening.
European Union foreign policy chief Catherine Ashton condemned the violence and urged Yemeni authorities to boost security at EU diplomatic missions in Sanaa.
"We have urged the Yemeni authorities to reinforce security of EU missions in Sanaa and to take the necessary measures to protect diplomats," Ashton added.
Violence also rocked the Egyptian capital Cairo, where police fired tear gas to disperse protests outside the embassy by stone- and bottle-throwing demonstrators.
A total of 224 people were injured, the Egyptian health ministry said.
On Tuesday night, protesters stormed the Cairo embassy compound, tearing down the Stars and Stripes and replacing it with a black Islamic flag.
-- 'Chaos' looms --
Egypt's Islamist President Mohamed Morsi condemned the offence caused by the movie but warned against resorting to violence.
"We Egyptians reject any kind of assault or insult against our prophet. I condemn and oppose all who... insult our Prophet," he said on state television.
"(But) it is our duty to protect our guests and visitors from abroad," Morsi said. "I call on everyone to take that into consideration, not to violate Egyptian law... not to assault embassies."
Amid the mounting protests, Russian President Vladimir Putin said the Middle East was at risk of descending into "chaos."
"We are afraid that the region may descend into chaos, which is essentially what is happening already," he said from his summer residence in the Black Sea resort of Sochi.
He also urged the new governments that rose to power in the Arab Spring uprisings to accept greater responsibility for security.
The low-budget movie in which actors have strong American accents, portrays Muslims as immoral and gratuitously violent.
US Secretary of State Hillary Clinton condemned the film, stressing that the US government had nothing to do with it.
"To us, to me personally, this video is disgusting and reprehensible. It appears to have a deeply cynical purpose, to denigrate a great religion and to provoke rage," Clinton said.
"The United States government had absolutely nothing to do with this video. We absolutely reject its content and message."
But she reiterated: "There is no justification, none at all, for responding to this video with violence."
State Department spokeswoman Victoria Nuland said all embassy staff in Yemen were safe and accounted for as White House spokesman Jay Carney insisted Washington was doing everything possible to protect its diplomats.
Saudi Arabia, home to Islam's holiest sites, condemned both the release of the amateur movie mocking Islam and the deadly attacks against US missions.
The Afghan government, meanwhile, ordered an indefinite ban on YouTube to prevent access to the film, officials in Kabul said.
Libya's new Prime Minister Mustafa Abu Shagur told AFP in an exclusive interview that there had been a "big advance" in the investigation into the attack on the US consulate in Benghazi.
Deputy Interior Minister Wanis al-Sharif said that arrests had been made but declined to elaborate on the number of people in custody or their backgrounds "so as not to hamper the smooth running of the investigation."
The Vatican condemned the Benghazi attack calling it an "organised" operation by "terrorist organisations."
Protests were also held in Iraq and Iran as anger over the US-produced movie "Innocence of Muslims" that initially erupted among Sunni Muslims spread to the two Shiite-majority countries.
Iran's supreme leader Ayatollah Ali Khamenei demanded the United States to punish those behind the film.
"If the American politicians are honest that they had no role, then they must punish those who committed this heinous crime and their financial backers in proportion to this great crime," he said, according to his official website.
In Kuwait about 500 demonstrators gathered near the US embassy waving black Al-Qaeda flags and chanting: "Obama, we are all Osama" bin Laden.