He called the killings an "outrageous attack" on Wednesday and promised to find the perpetrators, while ordering a tightening of diplomatic security worldwide.US Secretary of State Hillary Clinton condemned the killing as the work of a "small and savage group."Some media reports blamed an obscure anti-Islam film for the violence - killings in Libya, the invasion of the US Embassy in Cairo.But it appeared more likely it was the result of a plot by Islamists designed to attack the United States on Sept 11, the anniversary of the 2001 terrorist attacks on New York and Washington.US government officials said the Benghazi attack may have been planned in advance and there were indications that members of a militant faction calling itself Ansar al Sharia - which translates as Supporters of Islamic Law - may have been involved.They also said some reporting from the region suggested that members of Al-Qaeda's north Africa-based affiliate, known as Al Qaeda in the Islamic Maghreb, may have been involved."It bears the hallmarks of an organized attack," one US official said. However, some officials cautioned against assuming that the attacks were deliberately organized to coincide with the Sept 11 anniversary.Washington's envoy to Libya and three other Americans were killed during a mob attack on the the US consulate in Benghazi, sparking world outrage and drawing an apology from Libyan authorities on Wednesday.President Obama quickly ordered increased security at US diplomatic missions around the world, while slamming Tuesday's deadly assault in Benghazi, an Islamist stronghold in eastern Libya, that coincided with the anniversary of the September 11 attacks in the United States."I strongly condemn the outrageous attack on our diplomatic facility in Benghazi, which took the lives of four Americans, including Ambassador Chris Stevens," Obama said in a White House statement."I have directed my administration to provide all necessary resources to support the security of our personnel in Libya, and to increase security at our diplomatic posts around the globe," he added.Mitt Romney, Obama's rival in the November vote, criticised the president's response to the crisis. He said the timing of a statement from the US embassy in Cairo denouncing "efforts by misguided individuals to hurt the religious feelings of Muslims" made Obama look weak as protesters were attacking US missions.He said it was "disgraceful" to be seen to be apologizing for American values of free speech. Obama's campaign accused Romney of scoring political points at a time of national tragedy.A 50-strong unit US Marine counter-terrorism unit was en route to Libya to bolster security, a defence official said.US officials said the precise circumstances of the attack were still under investigation and that it was too soon to conclude exactly what transpired."It's too early to say," a US official, who asked not to be named, told AFP.Initial reports suggested the assault was triggered by outrage over an amateur, American-made movie that denigrates the Islamic faith. But subsequent speculation has raised the possibility of a deliberate plot by Islamist militants.Clips of the film at the centre of the controversy have been posted on the Internet and private satellite channels have been showing segments.The low-budget movie, "Innocence of Muslims", was produced by Israeli-American Sam Bacile, according to the Wall Street Journal.Stevens, a career officer with the US foreign service, had been in the country for less than four months after taking up his post in the capital Tripoli in May.Witnesses said he was killed when angry Islamists late Tuesday attacked the consulate with rocket-propelled grenades before looting and torching the building.A security source in Benghazi -- cradle of the 2011 uprising that toppled the regime of late dictator Muammar al-Gaddafi -- said it was suspected that the envoy may have suffocated due to carbon monoxide poisoning.A picture taken by an AFP photographer shows what witnesses say is an injured Stevens being aided by Libyans inside the premises of the consulate.A US official said five or six Americans were wounded in the attack.Libya's deputy ambassador to the United Nations, Ibrahim Dabbashi, said up to 10 Libyan security personnel were also killed or wounded.Obama paid tribute to the Libyans who had given their lives."Libyan security personnel fought back alongside Americans. Libyans helped some of the diplomats find safety and carried Ambassador Stevens's body to the hospital where we tragically learned that he had died," the US president said."The attack will not break the bonds between the United States and Libya."US Secretary of State Hillary Clinton said the killings should "shock the consciences" of people of all faiths but vowed that the actions of "a savage and small group" would not make Washington turn its back on Libya.The violence in Benghazi was strongly condemned by Libya's General National Congress, which nonetheless maintained it plans to elect a new prime minister at a session later on Wednesday.The first task for the new premier will be to bring order to the myriad of militias born out of last year's uprising."We present our apologies to the United States, the American people and the entire world for what happened," the GNC's president, Mohamed al-Megaryef, said in a statement.The United Nations also condemned the killings. "The United Nations rejects defamation of religion in all forms. At the same time, nothing justifies the brutal violence which occurred in Benghazi yesterday." a statement said.