Defence secretary Leon Panetta said the US military did not plan to bolster its forces in the Middle East and North Africa, despite the threat of more violent protests targeting diplomatic outposts across the region.
The spectre of renewed unrest after a relatively peaceful weekend grew with Hezbollah chief Hassan Nasrallah calling for a week of protests across Lebanon over the anti-Islamic internet video stirring up anger in the Muslim world.
"The whole world needs to see your anger on your faces, in your fists and your shouts," Nasrallah, the head of Lebanon's powerful Shiite Muslim organisation, said in a televised speech.
He spoke hours after Pope Benedict XVI left Lebanon following a historic three-day visit in which he prayed that Middle East leaders would work towards peace and reconciliation.
A low-budget trailer for a movie entitled Innocence of Muslims, believed to have been produced by a small group of extremist Christians in the US, has sparked furious anti-American protests across the Islamic world.
The first unrest was in Cairo, where protesters – reportedly stirred up by clips of the film presented on satellite TV channels and seen on YouTube – stormed the US embassy on Tuesday night, replacing the Stars and Stripes with an Islamic banner.
Hours later, the US consulate in Libya's eastern city of Benghazi came under sustained attack during more protests against the film – four Americans, including ambassador Chris Stevens, were killed in the assault.
Announcing the arrest of 50 suspects, Libya's Parliament chief blamed the attack on a few foreign extremists who he said entered Libya from Mali and Algeria and pre-planned it with local "affiliates and sympathisers".
"It was planned, definitely, it was planned by foreigners, by people who entered the country a few months ago," Mohammed al-Megaryef, president of the Libyan National Congress, told US broadcaster CBS television's Face the Nation.
Stevens is believed to have died from smoke inhalation after becoming trapped in the blazing diplomatic compound, which came under fire from rocket-propelled grenades, mortars and small arms for several hours.
US ambassador to the United Nations Susan Rice offered a different account of the Benghazi assault, saying it began with a "spontaneous" protest over the film and that there was no evidence of pre-planning.
Countering accusations of US impotence over the crisis, Rice said President Barack Obama had been extremely effective in getting Muslim leaders to protect US diplomatic facilities and condemn the violence.
"What we've seen is that the president has been incredibly calm, incredibly steady and incredibly measured in his approach to this set of developments," Rice said.
"His interventions, his leadership, has ensured that in Egypt, in Yemen, in Tunisia, in Libya and many other parts of the world, that leaders have come out and made very plain that there's no excuse for this violence."
Since the Benghazi consulate attack, the US has deployed counterterrorism Marine units to Libya to protect the Tripoli embassy and stationed two destroyers off the North African coast.
It has also sent a Marine unit to protect the US embassy in the Yemeni capital Sanaa, where police shot dead four protesters and wounded 34 others on Thursday as a mob stormed the facility and breached its perimeter.
The US has evacuated all non-essential staff and family members from Sudan and Tunisia and warned US citizens against travel to the two countries.
With a substantial force already deployed in the region and now boosted by extra Marine units, Panetta said the US military had the ability to respond as necessary to protect American diplomats.
"We do have a major presence in the region," he told reporters.
"Having said that, we've enhanced that with FAST [Fleet Anti-Terrorism Security Team] and others so that if they are requested, they can respond more quickly."
Protests targeting symbols of US influence, ranging from embassies and schools to fast food outlets erupted again on Sunday, with thousands rallying across Pakistan, burning American flags and effigies of Obama.
At least eight people were injured when protesters clashed with police outside the US consulate in the port of Karachi.
Hundreds of students poured onto the streets of Kabul shouting anti-American slogans, while the Bangladesh government condemned the film as "reprehensible" and New Delhi called it "offensive".
Belgian police said they detained 230 people in the northern city of Antwerp after clashes at a demonstration against the film.
In Afghanistan, heavily armed Taliban fighters on Friday stormed a strongly fortified air base in Helmand province where Britain's Prince Harry is deployed, killing two US Marines in an assault the militia said was to avenge the anti-Islam video.
A Nato spokesperson revealed that six US fighter jets and three refuelling stations were destroyed and six aircraft hangars damaged in the attack, the scale of which he said was unprecedented.
A total of 17 people have died in violence linked to the film, including the four Americans killed in Benghazi, 11 protesters who died as police battled to defend US missions from mobs in Egypt, Lebanon, Sudan, Tunisia and Yemen, and the two US soldiers in Afghanistan. – Sapa-AFP