Allied fighter jets have taken to the skies over Libya after leader Muammar Gaddafi apparently defied the UN's demand that he stop attacking his own people.
British ships are also involved in a naval blockade of the country, sources told Sky News, as the world launched a co-ordinated response based on an earlier UN resolution to protect Libyan civilians.
A French defence official has confirmed one of their fighter jets fired on a Libyan military vehicle.
The initiation of military action was revealed by Western leaders, following a summit in Paris to decide how to deal with what they said was the dictator's breach of a self-imposed ceasefire.
Reports say the rebel stronghold of Benghazi has been attacked by Gaddafi's militia, and the insurgents claim a captured warplane was shot down.
British Prime Minister David Cameron said: "The time for action has come and it needs to be urgent.
"Colonel Gaddafi has made this happen - he lied to the international community."
French President Nicolas Sarkozy added that military force would be used "in the absence of an immediate ceasefire" because "the Libyan people need our help."
Observers now believe selected targets in Libya will be hit within the next 24 hours.
Reports said Libyan forces intend to deploy 'human shields' to thwart any UN-backed bombing campaign.
State news agency Jana reported that civilians were converging on government facilities believed to be bombing targets for an impending UN-backed air intervention force.
Libyan capital of Tripoli under government restrictions, said: "I cannot independently confirm the claim of human shields but if it is true it is a worrying development that shows the regime will stop at nothing."
The rebel pilot of the fighter jet, which may have been shot down or suffered catastrophic engine failure, ejected moments it crashed in a fireball in Benghazi's southern suburbs.
Sky's Emma Hurd, who witnessed the crash, said: "It had been circling above the heavily populated areas and then it went into a fast dive and caught on fire."
A Benghazi resident named Sam later claimed the city was being hit by rocket fire from ground forces loyal to Libyan dictator Colonel Gaddafi.
She told Sky News: "Benghazi has been under continuous bombing since around 6am this morning - it was non-stop and the windows were shaking.
"Their troops have been bombing civilian areas with no military facilities... Civilians are being attacked in Benghazi."
But a government official insisted military forces were not being used to attack the city on Saturday, amid claims of 25 dead being taken to the city's hospitals.
Libya declared a ceasefire on Friday after the UN authorised the no-fly zone over the country.
Deputy foreign minister Khaled Kaim told Holland: "We are categorically denying there is any military operation on the ground (in the city of Benghazi) since we announced the decision has been made to cease fire."
Regime spokesman Ibrahim Moussa later said the Gaddafi's government remained defiant about the threat of military action.