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.
Christian group The attack on Camp Bastion in Helmand province, which continued into Saturday, 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 that 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.
Obama "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.
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 initial estimates ranged from 15 to 20 insurgents killed, but that it was not yet possible to confirm a final number. Prince Harry was not thought to have been affected.
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.
Helmand province in southern Afghanistan is one of the toughest battlegrounds of the Taliban's 10-year insurgency against some 117,000 NATO troops.
Camp Bastion is used by both US and British aircraft and is where Prince Harry, 27 -- third in line to the British throne -- has been deployed on a four-month tour as a helicopter pilot.
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.
Hundreds of Muslim protesters clashed with police in Sydney on Saturday as Australia became the the latest focus of disturbances.
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".
On Friday, clashes or demonstrations were reported from as far apart as Mauritania and Indonesia. Troops in Nigeria fired live rounds in the flashpoint city of Jos and Egyptian police fought street battles in downtown Cairo.
Tunisian demonstrators set fire to several vehicles and an American school during a failed attack on the main embassy compound, and in northern Lebanon, 300 Islamists set fire to a branch of the US fast food chain KFC.
In Washington, President Barack Obama welcomed home the bodies of the four US officials killed in Benghazi, Libya's second largest city and cradle of last year's Western-backed revolt which overthew and killed veteran dictator Moamer Kadhafi.
"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.
The unrest across the Muslim world clouded Pope Benedict XVI's three-day visit to Lebanon to promote Muslim-Christian coexistence.
European Union foreign policy chief Catherine Ashton on Friday urged leaders in Arab and Muslim countries to "call immediately for peace and restraint."
