The United States, Britain and France pounded targets in Libya with air strikes and Tomahawk missiles today, in a campaign to prevent Muammar Gaddafi from crushing a month-old uprising against his rule.

Libyan state television said a French plane was shot down, as an official in Tripoli denounced the "barbaric aggression" despite its announcement of a ceasefire in a month-long showdown against rebels.

US and British forces fired at least 112 Tomahawk cruise missiles against Libya's air defence sites, a top US military officer said, two days after a UN Security Council resolution with Arab backing authorised military action.

"Today, I authorised the armed forces of the United States to begin a limited military action in Libya," President Barack Obama said while on a visit to Brazil.

The military official said the first strikes aimed at imposing a no-fly zone took place near Libya's coast, notably around Tripoli and Misrata, "because that's where the integrated missile defence systems are."

French planes initially carried out four air strikes, destroying armoured vehicles, France's military said.

Libyan state media said Western warplanes bombed civilian targets in Tripoli, causing casualties, shortly after France's launch of the multinational air campaign against Kadhafi mounted with the support of the Arab League.

The strikes also hit fuel tanks feeding the rebel-held city of Misrata, east of Tripoli, a Libyan army spokesman said.

State television said hundreds of people had gathered at Bab al-Aziziyah, Gaddafi's Tripoli headquarters, and at the capital's international airport, ahead of the widely anticipated air strikes.

"Crowds are forming around the targets identified by France," the television reported, showing pictures of flag-waving people gathering to serve as human shields.

Last week, a highly placed French source referred to Bab al-Aziziya, a military air base in Sirte, east of the capital, and another in Sebha in the south as likely targets of a strike.

Britain also said its forces were in action, as Russia's foreign ministry expressed regret over the armed intervention under UN Resolution 1973 "which was adopted in haste."

According to France's army chief, a first French air strike took place around 1645 GMT against "a Libyan vehicle clearly identified as belonging to pro-Gaddafi forces".

Within the next hour, French Rafale and Mirage 2000 fighter jets conducted three other strikes, destroying armoured vehicles of the Libyan forces in the eastern region of Benghazi, the rebels' stronghold, the military said.

The operations are to continue through the night, the military said.

In the rebel camp, celebratory gunfire and honking of car horns broke out in Al-Marj, 100 kilometres from Benghazi, to welcome the start of military operations against Gaddafi, correspondents said.

As thousands fled Benghazi amid an assault by Gaddafi loyalists earlier today, French President Nicolas Sarkozy told a summit of world leaders in Paris that his country's fighters were poised to attack.

But Sarkozy said Gaddafi could still avoid the worst if he complied with the Security Council resolution by implementing a ceasefire to allow the diplomatic door to reopen.

UN chief Ban Ki-moon, however, said he was troubled by a telephone call from the Libyan prime minister last night.

"He told me that the Libyan government was fully abiding by the Security Council resolution and there will be an immediate ceasefire," said the secretary general.

"But at the same time and overnight they were attacking Benghazi. It is very troubling; whatever they say must be verified."

Since yesterday, the Libyan government has insisted it was observing a self-declared ceasefire, shortly after the Security Council voted to authorise the use of force against Gaddafi's troops to spare civilians.

The regime said its armed forces were under attack west of Benghazi, including by rebel aircraft, and had responded in self-defence.

Today, Foreign Minister Mussa Kussa said Tripoli had met all its obligations under the UN resolution and asked Ban to send observers to monitor the ceasefire.

But the rebels, who have been trying to overthrow the Libyan leader for more than a month, said government troops had continued to bombard cities, violating the ceasefire continuously.

British Prime Minister David Cameron held Gaddafi responsible for the continued violence and said that "the time for action" had come.

"Colonel Gaddafi has made this happen. He has lied to the international community, he has promised a ceasefire, he has broken that ceasefire. He continues to brutalise his own people," Cameron told British television.

"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."

Gaddafi has defied the threats against, telling Sarkozy and Cameron, main sponsors of the resolution authorising military action, that they would regret interfering in his country's affairs.

Earlier, a huge plume of smoke rose over Benghazi, Libya's second city, as thousands of people fled eastward after a series of air strikes and sustained shelling, said an AFP reporter in the metropolis of one million people.

Correspondents redeployed along with civilians to Al-Marj reported that Gaddafi tanks had entered the Mediterranean city by mid-morning.

A warplane crashed in flames in a residential area of Benghazi, triggering celebratory gunfire from the rebels, but an insurgent commander later admitted it was one of theirs and had been shot down by Gaddafi's forces.

Ban attended what host France said would be a "decisive" summit in Paris with US Secretary of State Hillary Clinton as well as countries in the European Union, Arab League and African Union.

Late yesterday, the French presidency said France, Britain, the United States and Arab countries demanded "that a ceasefire must be put in place immediately, that is, that all attacks against civilians must come to an end."

It added that "Gaddafi must end his troops' advance on Benghazi and withdraw from Ajdabiya, Misrata and Zawiyah," referring to rebel-held towns attacked or captured by government forces in past days.

"That is not negotiable," it said, warning that if Gaddafi did not comply with Resolution 1973, he would face "consequences" from the international community and "the resolution will be imposed by military means".

Speaking in Brazil, where he was on a visit today, US President Barack Obama said "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."

Rebels in Misrata said earlier that the city east of Tripoli was calm today, a day after they beat back an onslaught by Gaddafi forces, destroying heavy armour but suffering 27 casualties.

But a witness said government tanks had shelled rebel-held Zintan, 120 kilometres southwest of Tripoli.

In another Middle East hotspot, medics in Yemen on Saturday raised to 52 the death toll from a sniper attack on protesters in Sanaa the previous day, as thousands rallied despite a state of emergency.

The slaughter in Sanaa yesterday was the bloodiest day in weeks of unrest that have shaken the regime of President Ali Abdullah Saleh, a key US ally in its war against Al-Qaeda.

And security forces in Syria fired tear gas today at mourners burying two men killed in a protest in the southern city of Daraa the previous day, wounding several, rights activists said.

The official SANA news agency said a committee was being formed to investigate the "regrettable" events in Daraa.

In Bahrain, beleaguered King Hamad pledged to bring in reforms as Shiite-led pro-democracy protesters against the Sunni monarchy said they would not give up despite being cleared by police from Pearl Square in central Manama.