The White House said in a statement that US President Barack Obama spoke on Wednesday evening with his Libyan counterpart Mohammed el-Megarif, with the two leaders agreeing "to work closely over the course of this investigation."

"The president made it clear that we must work together to do whatever is necessary to identify the perpetators of this attack and bring them to justice," the statement said.

According to the White House, Obama also called Egyptian President Mohamed Morsi about protests in that country, and asked that he "cooperate with the United States in securing US diplomatic facilites and personnel."

Protests on September 11 - the anniversary of the 9/11 attacks in the United States - over an anti-Islamic film turned deadly in Libya's second city of Benghazi, with four Americans, including the US ambassador, being killed by insurgents late on Tuesday.

Uncertain motivation

US officials have, however, said that it was not clear if the attack was provoked by the film, an excerpt from which was posted on YouTube, or was a carefully planned operation by a terrorist group.

The Libyan Interior Ministry said on Wednesday that the mass protest outside the US consulate had been "manipulated" by people loyal to the regime of deposed dictator Moammar Gadhafi.

Among other things, the fact that the attackers apparently had knowledge of a safe house where two of the American diplomats were killed points to a premeditated assault.

Anti-American violence has hit other majority Muslim countries, with police using teargas to disperse protesters near the US embassy in Cairo after they scaled the walls. There have been protests at several other American missions, including in Bangladesh, Tunisia, Sudan, Morocco and Yemen.

The attack in Benghazi has provoked widespread international condemnation, and el-Megarif has apologized for the attack to the US government.

US domestic fallout

Republican challenger Mitt Romney has criticized the US president's reponse to the crisis, saying a statement from the embassy in Cairo denouncing "efforts by misguided individuals to hurt the religious feeling of Muslims" - issued before the attacks - had made Obama look weak.

Obama's campaign has accused Romney of trying to score political points at a time of national tragedy.

The 14-minute clip at the center of the anti-US protests is a trailer for a film called the "Innocence of Muslims," widely attributed to a man who described himself as a California-based Israeli Jew named Sam Bacile.

The video has been available on the Internet since July. However, it was only after a dubbed Arabic-language version appeared online last week that it began to arouse outrage in Muslim countries.

Google Inc., the owner of YouTube, says it has blocked acces to the clip in Libya and Egypt "given the very difficult situation" in those two countries.

tj/mkg (Reuters, dpa)