VATICAN CITY — The Vatican condemned on Thursday what it called the "terrorist" attack on the U.S. consulate in Libya in which the ambassador and three other American officials were killed.

"The very serious attack organised against the United States diplomatic mission in Libya . . . calls for the firmest possible condemnation on the part of the Holy See," said Vatican spokesman Federico Lombardi in a statement. "Nothing, in fact, can justify the activity of terrorist organisations and homicidal violence."

The statement came as Pope Benedict XVI prepares to travel to Lebanon Friday for a three-day visit on which he is expected to reiterate his calls for reconciliation in the Middle East.

Thursday's statement on the attack in Benghazi, Libya, is the Vatican's second in 24 hours.

In this latest statement, the emphasis shifted from the anti-Islamic film thought to have provoked the violence to the theory now being investigated that it may have been a pre-planned attack.

Tuesday's assault on the consulate was initially identified as a violent protest over an amateurish anti-Islamic film made in the United States.

But U.S. officials are now investigating the possibility that the assault was a plot by al-Qaida, using the protests as a diversion to carry out a coordinated attack on the 11th anniversary of the 9/11 attacks.

Protests against the film, which denigrates the prophet Mohammed, have spread across the Arab world and in Iran.