Libya attack: US to investigate Benghazi assaultBy BBC | Thursday, September 13  2012 at  11:42

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

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

US President Barack Obama has called the presidents of Egypt and Libya to urge cooperation over security for diplomatic staff.

There have been further clashes outside the US embassy in Cairo overnight.

Thousands of people gathered outside the building in the Egyptian capital, chanting and starting fires.

Police and troops fired tear gas to break up the crowds, some of whom were seen carrying petrol bombs.

Tight security

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

President Barack Obama has vowed to bring those behind the attack to justice, but said the US would not "break bonds" with the new Libyan government.

The White House says Mr Obama spoke to Libyan President Mohamed Magarief on Wednesday evening and agreed the two countries should work together to identify the attackers.

In a further call to Egypt, President Mohammed Morsy promised to ensure the safety of American staff in the country, the White House said.

Security has been tightened at US embassies across the world.