Washington said it was deploying forces to cope with violence in as many as 18 different locations as deadly Muslim anger spreads over a US-made movie that mocks Islam.
Two US marines were killed in Afghanistan when insurgents armed with guns and rockets stormed a heavily fortified air base late on Friday in an attack that the Taliban militia said was to avenge the film.
The attack on Camp Bastion in Helmand province, which continued until Saturday morning, was a major security breach at a base where Britain's Prince Harry is stationed and has been the target of specific death threats.
It came after at least six protesters died in Egypt, Tunisia, Lebanon and Sudan on Friday as local police battled to defend American missions from mobs of stone-throwers.
Symbols of US influence in cities across the Muslim world came under attack -- embassies and schools as well as fast food chains -- as protesters vented their fury at the low-budget American-made YouTube film, "Innocence of Muslims".
US Defence Secretary Leon Panetta said Washington was configuring its forces to be able to cope with widespread violence following its deployment of Marine counter-terrorism units to Libya and Yemen and its stationing of two destroyers off the North African coast.
"We have to be prepared in the event that these demonstrations get out of control," Panetta told Foreign Policy magazine.
He did not offer any specifics. But the magazine said that the Pentagon was discussing, but had not yet decided, whether to send a third platoon of 50 specially trained Marines to protect the US embassy in Khartoum.
Guards on the roof of the embassy fired warning shots on Friday as the compound was breached by protesters waving Islamic banners, after earlier ransacking parts of the British and German missions in the Sudanese capital.
The US embassy compounds in Egypt and Yemen have also been breached in the past week, and on Tuesday the US ambassador to Libya, Chris Stevens and three other Americans were killed when a mob torched the consulate in Benghazi.
Panetta said on Friday that it was still too early to say exactly what happened in Benghazi where there have been suggestions that Al-Qaeda sympathisers rather than angry Muslim protesters may have been responsible.
"It's something that's under assessment and under investigation, to determine just exactly what happened here," he said.
The assault on the consulate came on the anniversary of the September 11, 2001 attacks in the United States and the head of Libya's national assembly, Mohammed al-Megaryef, on Friday blamed Al-Qaeda as he laid a bouquet of flowers in front of the devastated mission.
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In Friday night's attack in Afghanistan, the assailants managed to penetrate the air base and damaged several aircraft although military spokesman Major Adam Wojack declined to say what type or how many.
Wojack said 18 insurgents were killed -- including a suicide bomber. Prince Harry was never in danger, officials confirmed.
A Taliban spokesman said the attack was to avenge the YouTube movie.
"Last night, a number of mujahedeen fighters have carried out suicide attacks on Camp Bastion of Helmand in revenge for the insulting movie by the Americans," spokesman Qari Yousuf Ahmadi told AFP by telephone.
Britain's Ministry of Defence said in June that the massive base, spread over several square kilometres (miles) of desert, is home to more than 28,000 people and the attack raised serious questions about security.
Police in Sydney fired pepper spray to contain protesters trying to enter the building housing the US consulate on Saturday, as Australia became the the latest focus of disturbances.
Bottles, shoes and other objects were hurled during the clashes with police which resulted in eight arrests, with six police officers injured as the unexpected protest brought parts of the city to a standstill.
Shoppers looked on in surprise as protesters, including children, shouted "Down, down USA" and waved banners such as "Behead all those who insult the prophet".
Hundreds also demonstrated in Indonesia and the Maldives.
In Somalia, the Al-Qaeda-linked Shebab militia, which controls large swathes of the country called on Muslims to launch revenge attacks on Western targets.
"The Shebab mujahedeen are urging people of Somalia to show their love for Islam and particularly to our Prophet Mohammed by making attacks against the West," Shebab spokesman Ali Mohamud Rage told AFP by telephone.
US President Barack Obama urged Americans not to be disheartened by images of anti-American violence in the Islamic world, expressing confidence that the ideals of freedom America stands for would ultimately prevail.
"I know the images on our televisions are disturbing," Obama said in his weekly radio and Internet address. "But let us never forget that for every angry mob, there are millions who yearn for the freedom, and dignity, and hope that our flag represents."
Obama said his administration was doing everything it could to protect Americans serving abroad.
"We are in contact with governments around the globe, to strengthen our cooperation, and underscore that every nation has a responsibility to help us protect our people," he said.