The US is investigating whether the attack in Libya that killed the US ambassador and several other people was planned in advance, officials say, - BBC reported.

They said the US consulate attack was complex and professional, amid reports the perpetrators may have had links to jihadist groups.

The assault had earlier been thought to have been a spontaneous reaction to protests over an anti-Islamic film.

There were also clashes outside the US embassy in Cairo on Thursday.

Thousands of people gathered outside the building in the Egyptian capital for a second night, chanting and starting fires. Police and troops fired tear gas to break up the crowds, some of whom were seen carrying petrol bombs.

On Wednesday, demonstrators in Cairo angry at the film - called Innocence of Muslims - had breached the walls of the US embassy and torn down the flag.