President Barack Obama Friday delivered a frank ultimatum to Moamer Kadhafi, threatening military action if he ignores non-negotiable demands for a ceasefire and a retreat from rebel bastions.

US President Barack Obama speaks during a statement on Libya in the East Room of the White House in Washington, DC.

The US warning was immediately backed by France, Britain and Arab nations which have driven demands for a no-fly zone and other measures to save civilians in Libya, now enshrined in a UN resolution mandating action.

The US president made dramatic and forceful on-camera demands of Kadhafi, saying the world could not stand by because, if left unchecked, the Arab strongman would commit atrocities in which thousands would die.

But Obama left unclear the exact role that Washington would play in military action against Libya, beyond enabling its European and Arab partners to "effectively" mount a no-fly zone and a mission to protect civilians.

He made clear, however, that there would be no US ground troops deployed to Libya, and repeatedly stressed that the United States, which is still stung by the price of its 2003 invasion of Iraq, would not take unilateral action.

"The United States is prepared to act as part of an international coalition. American leadership is essential, but that does not mean acting alone.

"It means shaping the conditions for the international community to act together."

"Our goal is focused, our cause is just, and our coalition is strong."

Obama spoke as opposition rebels in the stronghold of Benghazi and other cities awaited Kadhafi\'s response, after it said earlier it would observe a ceasefire.

A loud explosion and anti-aircraft fire was heard late on Friday in the city, AFP correspondents said, though the cause was not immediately known.

Obama said that if Kadhafi failed to honor the UN Security Council resolution passed Thursday, demanding a complete ceasefire among other requirements, he would face "consequences."

"These terms are not subject to negotiation. The resolution will be enforced through military action."

"Left unchecked, we have every reason to believe that Kadhafi would commit atrocities against his people," Obama said. "Many thousands could die. A humanitarian crisis would ensue."

But Obama significantly did not demand Kadhafi step down, but argued he had lost legitimacy and the confidence of his own people.

Many of the possible military actions which the coalition could take however, would surely increase pressure on Kadhafi\'s rule, and Washington has left no doubt that it would prefer its long-time enemy gone.

The president warned Kadhafi had shown a willingness to use "brute force" throughout his four decades in power, and noted the Libyan leader had pledged as recently as Thursday to show "no mercy and no pity" for his own citizens.

Obama also laid down new and specific, conditions for Kadhafi, saying he must immediately halt his advance on the key rebel town of Benghazi and pull back from three other rebel contested cities, or face military action.

"That means all attacks against civilians must stop," Obama said.

"Kadhafi must stop his troops from advancing on Benghazi, pull them back from Ajdabiya, Misrata and Zawiyah and establish water, electricity and gas supplies to all areas," Obama said.

In a statement issued by the French presidency soon after Obama spoke, France, Britain and Arab nations joined Washington in calling on Kadhafi to "immediately" end attacks against his people or face military force.

The countries said "a ceasefire must be put in place immediately" adding that attacks against civilians must end.

Mirroring Obama\'s warning, the statement warned "Kadhafi must end his troops\' advance on Benghazi and withdraw from Ajdabiya, Misrata and Zawiyah.... That is not negotiable."

Obama said he would also send Secretary of State Hillary Clinton back to Europe to attend a major summit in Paris on Libya on Saturday, which includes the leaders of France, Britain and Germany as well as Arab allies.

He also held consultations at the White House with key members of Congress, outlining his plan of action.

"Kadhafi has lost legitimacy and it is only a matter of time before he is out of power," said Democratic Senate Majority leader Harry Reid.

"Together with our allies, we should work to ensure that Kadhafi\'s transition out of power is swift, and that the people of Libya do not suffer any more than they already have."

Another of Obama\'s Democratic allies, Senator John Kerry added: "President Obama\'s stern ultimatum to Kadhafi is the right message. There must be a full cessation of hostilities immediately."