The United States and Britain have tonight launched air strikes against the Afghan capital, Kabul.

As explosions and anti-aircraft fire were heard in the city, President Bush's spokesman Ari Fleischer said: "We are beginning another front in our war against terrorism so freedom can prevail over fear."

The first explosions could be heard about 1727 UK time. Electricity was shut off throughout the city.

Five large explosions shook Kabul, followed by the sounds of anti-aircraft guns.

Power went off almost immediately after the first of five thunderous blasts.

The blasts, which appeared to have been in the south-west of the city.

In an address to the US nation, Mr Bush said the war on terrorism was under way and would be targeted to disrupt Afghanistan's role as a terrorist base.

He said: "The battle is now joined. We will not waver, we will not falter, we will not fail.

"We did not ask for this mission, but we will fulfil it."

Mr Bush added: "Every nation has a choice to make in this conflict. There is no neutral ground."

UK Prime Minister Tony Blair said British missile-firing submarines were involved in tonight's attack and other forces would be available for future operations.

Mr Blair praised the "courageous" British armed forces whose "sense of duty" is renowned "throughout the world".

He said the Taliban regime had been given the chance to surrender bin Laden, and added: "It's clear beyond doubt that they will not do this.

"They were given the choice to side with justice or side with terror and they decided to side with terror."

The Islamabad-based Afghan Islamic Press agency quoted the Taliban as saying planes had bombed areas near Kabul airport in the northern part of the city.

The Taliban's consul-general in the Pakistani city of Karachi said tonight: "We have received word that a plane has bombed Kabul. We don't have any more information. We are ready for jihad (holy war)."

US officials said the attack on Kabul was actually carried out by Tomahawk cruise missiles.