A series of more than 110 cruise missile strikes on Libyan targets is only the first phase of a multiphase operation, a senior U.S. military official said on Saturday.
Vice Admiral Bill Gortney, director of the U.S. military's Joint Staff, declined to discuss what the next phase of the operations would be. He said more than 20 sites had been targeted in coalition strikes so far.
“We are on the leading edge of a coalition military operation," Gortney said. "This is just the first phase of what will like be a multiphase operation."
But he said it will be six to 12 hours before commanders, using Global Hawk unmanned aircraft and other means, can assess the damage the strikes have inflicted.
Gaddafi has warned Western powers they will regret getting involved in his country's internal affairs.
The United States, France, Britain, Canada and Italy make up the coalition carrying out the operation, called "Odyssey Dawn." Arab nations are expected to join later, said a U.S. military official on condition of anonymity.
Some 25 coalition ships, including three U.S. submarines armed with Tomahawk missiles, are stationed in the Mediterranean, a military slide showed. Five U.S. surveillance planes are also in the area.
Officials said U.S. forces and aircraft will take part but U.S. planes had not yet begun patrolling the skies above Libya.
Officials said General Carter Ham, head of U.S. Africa Command, was taking the lead in the operation.
Gortney said no U.S. forces were on the ground in Libya, but the United States would take a leading role in the operation's logistics, such as refueling.
Libyan state media Western warplanes bombed civilian targets in Libya's capital on Saturday, causing casualties,
"Civilian targets are being bombed by the 'Crusader' enemy fighter planes in Tripoli," state television said.
State news agency Jana said there were "civilians casualties as a result of this aggression."
Earlier witnesses told AFP that several loud blasts were heard east of Tripoli and that "balls of fire" could be seen on the horizon. "We don't know where the explosions occurred," a resident of Tripli's eastern suburb said.
The French air force earlier destroyed some tanks and armored vehicles of troops loyal to Gaddafi that were threatening the civilian population, the French military said, hours after President Nicolas Sarkozy announced the start of the military operations in the North African state.
"The vehicle was clearly identified as being enemy," army spokesman Colonel Thierry Burckhard said after the first UN-mandated air strike, describing the target as "a vehicle that was threatening the civilian population".
Sarkozy had said allied air forces had gone into action on Saturday evening over Libya and were preventing Gaddafi's forces attacking opposition fighters and civilians.
"Our planes are already preventing air attacks on the city (Benghazi)," he said, adding that military action supported by France, Britain, the United States and Canada and backed by Arab nations could be halted if Gaddafi stopped his forces attacking.
British Prime Minister David Cameron told British television reporters after a meeting of international leaders in Paris that Gaddafi has broken the ceasefire and will face urgent action to prevent more civilian deaths
"Colonel Gaddafi has made this happen. He has lied to the international community, he has promised a ceasefire, he has broken that ceasefire," Cameron said.
"He continues to brutalise his own people and so the time for action has come. It needs to be urgent, we have to enforce the will of the United Nations and we cannot allow the slaughter of civilians to continue."
Obama warns Gaddafi
U.S. President Barack Obama warned Libyan leader Muammar Gaddafi on Saturday that the international community would act with urgency to shield Libyan rebels from his aggression unless the violence stopped.
"The international community demanded an immediate ceasefire in Libya, including an end to all attacks against civilians," Obama said during an appearance with Brazilian President Dilma Rousseff.
"Our consensus was strong and our resolve is clear. The people of Libya must be protected and in the absence of an immediate end to the violence against civilians, our coalition is prepared to act and act with urgency," he said.
German Chancellor Angela Merkel said world powers were united that violence in Libya must end, but confirmed her country would take no part in military action.
She made the comment after world leaders discussed how to coordinate a military intervention in Libya in Paris.
Berlin abstained on Thursday in a U.N. Security Council resolution authorizing a no-fly zone over Libya and ruled out any military involvement.
However, Merkel noted that her government has offered German AWACS aerial reconnaissance troops could take over duties in Afghanistan, freeing up U.S. forces to do the same job over Libya.
A french military source earlier said that several French Rafale fighter jets on Saturday overflew "all Libyan territory" on reconnaissance mission, ready to enforce an air-exclusion zone to halt Gaddafi’s attacks.
The planes were in the air not far from Libya this afternoon and were ready to carry out air strikes if there are orders from Sarkozy, a military official with knowledge of the preparations said on condition of anonymity.
Gaddafi continues violence
A witness told Al Arabiya television on Saturday that Zintan in western Libya was being bombarded and Libyan leader Muammar Gaddafi's tanks were approaching the town. "Now we are being bombed in Zintan from more than one direction: from the north and the south," said the witness, who was not identified.
"There are tanks heading towards the southern entrance of Zintan, around 20 to 30 tanks, which are hitting the city and residential areas in the south," he said.
The witness added that Gaddafi forces were attempting to subdue the opposition-held town because of its strategic location west of the capital Tripoli.
Gaddafi's forces also battled insurgents on the outskirts of the opposition-held city of Benghazi on Saturday, defying world demands for an immediate ceasefire and forcing opposition fighters to retreat.
The advance by Gaddafi's troops into Libya's second city of 670,000 people appeared to be an attempt to pre-empt Western military intervention which diplomats say will come after an international meeting currently underway in Paris.
A Libyan opposition spokesman said Gaddafi's forces had entered Benghazi while a Reuters witness saw a jet circling over the city shot down and at least one separate explosion near the opposition movement's headquarters in the city.
"They have entered Benghazi from the west. Where are the Western powers? They said they could strike within hours," opposition military spokesman Khalid al-Sayeh told Reuters.
Hundreds of cars full of refugees headed east from Benghazi towards the Egyptian border. One family of 13 women from a grandmother to small children, fled Benghazi on Saturday morning.
"I'm here because when the bombing started last night my children were vomiting from fear," said one of them, a doctor, sitting crying in the lobby of a hotel on the road to Egypt. "All I want to do is get my family to a safe place and then get back to Benghazi to help. My husband is still there."
In the besieged western city of Misrata, residents said government forces shelled the opposition town again on Saturday and they were facing a humanitarian crisis as water supplies had been cut off for a third day.
"I am telling you, we are scared and we are alone", a Misrata resident, called Saadoun, told Reuters by telephone.
Gaddafi said Western powers had no right to intervene.
"This is injustice, this is clear aggression," government spokesman Mussa Ibrahim quoted Gaddafi as saying in a letter to France, Britain and the United Nations. "You will regret it if you take a step towards interfering in our internal affairs."
The Libyan government blamed the opposition fighters, who it says are members of al Qaeda, for breaking the ceasefire around Benghazi.