French warplanes have hit four tanks used by forces loyal to Muammar Gaddafi on the outskirts of the opposition stronghold of Benghazi, on a day when opposition fighters in the city reported coming under constant artillery and mortar fire.
The US has also launched cruise missiles from ships based in the Mediterrannean Sea, and has taken the lead in military operations during the first phase, Kimberly Halkett, Al Jazeera's correspondent in Washington DC reported.
The action marks the first international military moves against the Libyan leader, and it comes a day after the UN Security Council authorised a no-fly zone over the North African country.
In the first phase of the operation, named "Odyssey Dawn", the US will target Libyan Integrated Missile Defence Systems, mainly near Tripoli and Misurata, officials said. French aircraft had initiated operations by targetting Libyan air fields.
A spokesman for the French military had confirmed that his country's fighter jets have attacked a vehicle belonging to Gaddafi's forces.
Alain Juppe, the French foreign minister, said that operations "will continue over the next few days" until the Gaddafi government complies with UN Resolution 1973.
Libyan state television reported that civilian targets in Tripoli had been bombarded, as well as fuel stores in Misurata. The state news agency reported that there had been "civilian casualties as a result of this aggression".
In a brief statement on state television, Muammar Gaddafi said that the air strikes marked the beginning of another "crusade", adding that the Mediterranean and North Africa were being turned into a "battleground".
Mohammad al-Zawi, the secretary-general of the Libyan parliament, told reporters late on Saturday that his country was facing a "barbaric" attack, and reiterated that Libyan forces had been observing a "ceasefire".
The US says that no coalition military operations are currently planned for near the opposition stronghold of Benghazi, but that a no-fly zone will include Tripoli, Sabha, Natoura, Misurata and Benghazi.
Earlier on Saturday, pro-government forces had entered the western outskirts of the city, while French Rafale and Mirage fighter jets began reconnaissance overflights of "all Libyan territory".
Meanwhile, four Al Jazeera journalists were detained in western Libya on Saturday. It was not immediately clear what charges, if any, they were being held on in Tripoli.
Fighting in Benghazi
Witnesses in Bengazi, in the east of the country, said they heard large explosions on Saturday. Al Jazeera's correspondents in the city reported multiple explosions, plumes of smoke in the sky and a fighter jet belonging to the opposition getting shot down
Government troops reportedly bombed the southern Benghazi suburb of Goreshi among other places. Artillery and mortars were also fired in the centre of the city.
Mustafa Abdel Jalil, head of the opposition National Libyan Council, told Al Jazeera "there is a bombardment by artillery and rockets on all districts of Benghazi".
