America, caught by surprise by the ferocious September 11 attack against the US consulate in Libya that killed the US ambassador and three other countrymen, is bracing itself for more potential violence in parts of the Muslim world today.

Fears are growing that another attack will come after Friday's weekly prayers - traditionally a time of protest in the Middle East and North Africa.

Angry demonstrations over an anti-Islam video have already occurred in Egypt and Yemen and officials believe well-armed Libyan extremists hijacked a similar protest in Benghazi, where several Libyan security guards were also killed.

The US put all of its diplomatic missions overseas on high alert and secretary of state Hillary Clinton delivered an explicit denunciation of the video as the administration sought to pre-empt further turmoil at its embassies and consulates.

"The United States government had absolutely nothing to do with this video," she said yesterday, before a meeting with the foreign minister of Morocco at the US State Department. "We absolutely reject its content and message.

"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."

US officials said they suspect that the attack at the Benghazi consulate, which had also been the target of an unsuccessful attack in June, may have been only partially related to the film.

They also stressed there had been no advance warning or intelligence to suggest a threat in Libya that would warrant boosting security, even on the 11th anniversary of the terror attacks of September 11 2001.

"As we did with all of our missions overseas, in advance of the September 11 anniversary and as we do every year, we did evaluate the threat stream and we determined that the security at Benghazi was appropriate for what we knew," State Department spokeswoman Victoria Nuland said.

The intelligence leading up to the attacks will be examined to "see if there was any way of forecasting this violence", as in any violent incident, House of Representatives Intelligence Committee member Adam Schiff said. But he said the focus now "has to be on finding out who is responsible and bringing them to justice".

President Barack Obama, speaking a campaign event in Golden, Colorado, also vowed that the perpetrators would be punished.

"I want people around the world to hear me," he said. "To all those who would do us harm: No act of terror will go unpunished. I will not dim the light of the values that we proudly present to the rest of the world. No act of violence shakes the resolve of the United States of America."

As of last night, there was no intelligence indicating that what happened in Benghazi was planned, according to two US officials briefed on the investigation into the attack. Intelligence chiefs believe it is more likely that the attack was "opportunistic or spontaneous", with militants taking advantage of the demonstration to launch the assault.

There is also no evidence that the attack was tied to 9/11, one of the officials said. But the Libyan-based militant group Ansar al Sharia is the leading suspect for carrying out the violence, possibly with help from al Qaida's main African-based offshoot, Al Qaida in the Islamic Maghreb.

Libya arrested an unspecified number of militants suspected of taking part in the attack and authorities were closely monitoring others to see whether they were linked to a group.

Despite the belief that the militants who hit the consulate did so separately from the protests over the film, US officials are deeply concerned that extremists may take advantage of non-violent demonstrations to copycat the Benghazi raid, or that otherwise peaceful protesters may be incited to attack because of the video, particularly today.

It was virtually impossible to predict when a crowd might form and turn violent, said retired US ambassador John Negroponte, who served at five posts, including Iraq, and was a former director of national intelligence.

"These things can be mobilised on the spur of the moment, set off by a spark," especially in places such as Egypt and Libya where the ruling strongmen have just fallen, he said.

"When you get rid of authoritarian regimes, there's little or no institutional framework left. ...That's why there's disorder and chaos" that is so easily hijacked.

United Nations secretary general Ban Ki-moon condemned the violence and also the provocative video. "Nothing justifies such killings and attacks," he said, adding that he also condemned "the hateful film that appears to have been deliberately designed to sow bigotry and bloodshed".

Egypt's ruling Muslim Brotherhood called for demonstrations after Friday prayers as did authorities in Iran and the Gaza strip. Large protests were expected in Baghdad and Iraq's second-largest city, Basra, as well as Amman, Jordan. Israel was stepping up security in anticipation of demonstrations after Muslim prayers.

The White House said it was prepared for more protests but stressed that any violence would be unjustified.

"It is important to note that as these protests are taking place in different countries around the world, responding to the movie, that Friday has historically been a day when there are protests in the Muslim world," White House press secretary Jay Carney said. "And we are watching very closely for developments that could lead to more protests. We anticipate that they may continue."

Indonesian leaders and a prominent cleric urged calm in the world's largest Muslim majority nation ahead of expected protests.

Jakarta police spokesman Colonel Rikwanto said the US embassy in Jakarta did not request any increased security, but 250 riot police had been put on alert.

The embassy issued an email to American citizens saying a demonstration with an estimated 300 people was expected after Friday prayers.