Military action against Libyan forces was under way today as Prime Minister David Cameron declared that “the time for action has come” after dictator Muammar Gaddafi flouted his own ceasefire to mount continued attacks against his own people.

Mr Cameron joined other world leaders – including representatives of several Arab states – at an emergency summit in Paris which agreed to deploy military aircraft to stop the assault on rebel stronghold Benghazi.

French President Nicolas Sarkozy announced that “as of now” his country’s air force was operating in the skies over Libya, while further French jets stood ready to target Gaddafi’s tanks and armoured vehicles.

It was not immediately clear how soon the RAF’s Typhoon and Tornado fighters will go into action, though the Dutch Prime Minister was reported to have said that UK, US and French planes could be deployed later today.

Despite a ceasefire announced yesterday by Gaddafi’s regime, the bombardment of opposition-held cities Benghazi, Misrata and Ajdabiya continued today.

Mr Cameron said it was clear that Gaddafi had violated the ceasefire and action was now needed “urgently”.

“What is absolutely clear is that Gaddafi has broken his word, broken the ceasefire and continues to slaughter his own civilians,” said the Prime Minister after the summit ended.

“This has to stop. We have to make it stop. We have to make him face the consequences. So I think it is vitally important that action takes place and action takes place urgently.

“Obviously at this time our thoughts should be with those who are going to risk their lives to help save the lives of others.

“I think it is vitally important that, with the United Nations behind us, with the clear legality of this action, and with local countries supporting us as well, it is right to act.”

Mr Cameron added: “Of course there are dangers, there are difficulties. There will always be unforeseen consequences of taking action, but it is better to take this action than to risk the consequences of inaction, which is the further slaughter of civilians by this dictator flouting the United Nations and its will.”

Asked if military action would now go ahead Mr Cameron added: “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 people, and so the time for action has come.

“It needs to be urgent action. We must enforce the will of the UN. We cannot allow the slaughter of civilians to continue.”

Alongside Britain and France, today’s summit at the Elysee Palace was attended by UN Secretary General Ban Ki-moon, US Secretary of State Hillary Clinton, Arab League Secretary General Amr Moussa, EU President Herman van Rompuy and representatives of Arab states including Qatar, the United Arab Emirates, Iraq, Jordan and Morocco.

Italian Prime Minister Silvio Berlusconi and Spain’s Jose Luis Zapatero – who have both offered the use of air bases on their territory – were also attending, along with Germany, Denmark, Norway, Belgium, Canada, Greece, the Netherlands and Poland.

Mr Sarkozy said it had been agreed to take action to enforce UN Security Council resolution 1973, passed on Thursday, which authorised “all necessary measures” short of foreign occupation to protect civilians in Libya.

Added urgency was given to the gathering by reports that forces loyal to Gaddafi were storming the city of Benghazi, cradle of the month-long uprising against his 42-year rule.

A warplane seen crashing in flames outside the city was reported to belong to the forces of the opposition and may have been shot down by Gaddafi’s troops.

There were reports on Libyan state television of civilians massing as “human shields” at locations thought to be possible targets for allied air strikes.

Speaking at the end of the summit, Mr Sarkozy said: “Our air force will oppose any aggression by Colonel Gaddafi against the population of Benghazi.

“As of now, our aircraft are preventing planes from attacking the town. As of now, other French aircraft are ready to intervene against tanks, armoured vehicles threatening unarmed civilians.”

Participants in the summit had agreed “to use all necessary means – in particular military means – to enforce the Security Council decisions”, he said.

Mr Sarkozy said that Gaddafi had brushed off calls for an immediate ceasefire and withdrawal of his troops.

He explained: “Colonel Gaddafi has totally ignored this warning. In the last few hours his forces have stepped up their deadly offensives.

“Arab peoples have chosen to free themselves from the enslavement in which they have felt trapped for too long.

“These revolts have given rise to great hopes in the hearts of all those who share the values of democracy and human rights.”

The French President said that the allies had a duty to provide the Libyan people with help and support.

He added: “The future of these Arab peoples belongs to them. Amidst the many difficulties and ideals they must confront, these Arab peoples need our help and support and it is our duty to provide it.

“In Libya a peaceful civilian population demanding open law and the right to chose its own destiny is in mortal danger. It is our duty to respond to their anguish.

“The future of Libya belongs to the Libyans, we do not seek to decide for them. Their fight for freedom is theirs.”

He added that the intervention of the allies was not with a view to imposing a “specific” outcome on them.

Mr Sarkozy said Gaddafi’s regime had forfeited all legitimacy but insisted that it was still not too late for the Libyan leader to avoid “the worst”.

He added: “Today we are intervening in Libya under the United Nations Security Council mandate, alongside our partners and our Arab partners.

“We are doing this in order to protect the civilian population from the murderous madness of a regime that by killing its own people has forfeited all legitimacy.

“We are intervening in order to enable the Libyan people to chose its own destiny.

“It must not be deprived of its rights by violence and terror.

“There is still time for Colonel Gaddafi to avoid the worst by complying immediately with and unreservedly with all the demands of the international community.

“The doors of diplomacy will open once again when the aggression stops.”

Gaddafi himself defiantly dismissed the UN resolution as “invalid” and warned that Britain would “regret it” if he intervened in Libya.

In an open letter to Mr Cameron, Mr Sarkozy and Mr Ban released in capital Tripoli today, Gaddafi said: “Libya is not yours. Libya is for the Libyans. The Security Council resolution is invalid.

“You will regret it if you dare to intervene in our country.”

The Libyan leader was more conciliatory in a separate message to US President Barack Obama, asking him: “If you found them taking over American cities by the force of arms, tell me, what you would do?”

In Tripoli, Libya’s oil ministry urged Western firms which abandoned operations in the country at the outbreak of unrest last month to return, warning that contracts may otherwise be handed over to companies from countries such as China and India, which did not back military action.

Mr Cameron yesterday said RAF Typhoons and Tornados, together with surveillance and air-to-air refuelling craft, would take part in the international effort to prevent Gaddafi committing a “bloodbath” against his own people.

The Ministry of Defence this morning declined to confirm whether any RAF planes had set off on their mission, codenamed Operation Ellamy, or where they would be based in the Mediterranean.

Canadian military planes landed at Prestwick Airport in Ayrshire today to refuel.

An airport spokesman said: “Prestwick is often used by military aircraft to refuel, that happens throughout the year.

“Military aircraft land to refuel and move on, so it’s quite normal.”

Libya’s deputy foreign minister Khaled Kaim this morning told the BBC that the regime’s ceasefire was “real, credible and solid” and repeated an invitation for foreign observers to monitor it.

Mr Kaim insisted that Gaddafi’s air force had been grounded in response to Resolution 1973.

But he warned Libya would fight any attempt to intervene militarily in its territory. And he said foreign military intervention would suck countries from across the Arab world into the conflict.

“If there is an attack from outside or foreign intervention, you will see not only Libyans fighting foreign intervention, you will see people from Algeria, from Tunisia, from Egypt – and by the way Libyan tribes extend beyond Libya’s borders to Chad and Niger and Mali. All of them will be part of fighting against any foreign troops on Libyan ground,” said Mr Kaim.