WASHINGTON – President Barack Obama Monday told Americans his actions had stopped a "massacre" in Libya, but warned a military campaign to oust Moamer Kadhafi could repeat the bloodshed and misery of Iraq.

Obama mounted a firm defense of his decision to launch air strikes and launch a no-fly zone as part of an international coalition to protect civilians after the teetering Arab strongman threatened his own people with a bloodbath.

He justified the operation as vital to honoring US "interests and values," rejected claims he had been too slow to act, and made clear to war-weary Americans that the future US role would be limited.

"In just one month, the United States has worked with our international partners to mobilize a broad coalition, secure an international mandate to protect civilians, stop an advancing army, prevent a massacre, and establish a no-fly Zone with our allies and partners," Obama said in a televised speech.

The US leader said he had no choice but to act with international partners after Kadhafi rejected an offer to stop his "campaign of killing" and his forces surged towards the key city of Benghazi.

"Kadhafi declared that he would show 'no mercy' to his own people. He compared them to rats, and threatened to go door to door to inflict punishment," Obama said.

"I refused to let that happen."

Obama admitted that there was "no question" that Libya and the world would be better off if Kadhafi went, and vowed to pursue his ouster in a manner that stopped short of a military bid to eject him from power.

"If we tried to overthrow Kadhafi by force, our coalition would splinter," he warned.

"We would likely have to put US troops on the ground, or risk killing many civilians from the air.

"To be blunt, we went down that road in Iraq.

"Regime change there took eight years, thousands of American and Iraqi lives, and nearly a trillion dollars. That is not something we can afford to repeat in Libya."

As international powers begin to contemplate the task of framing a political future in Libya without Kadhafi, Obama warned that they faced a "difficult task."

"Forty years of tyranny has left Libya fractured and without strong institutions," Obama said, calling on the international community to join the Libyan people to build their future.

The critical address came on the eve of a major international conference on next steps on Libya in London, and as Libyan rebels, aided by coalition bombardments, make quick gains against loyalist forces.

Obama has faced a rising storm of criticism in recent days, especially from lawmakers who argued they were not fully consulted on the operation.

Congress has the sole constitutional power to declare war, but Obama aides said they had no time to request authorization as the humanitarian crisis was so acute, and maintain the operation in Libya falls short of full scale war.

"If the American people are uncertain as to our military objectives in Libya, it's with good cause," said Mitch McConnell, the top Senate Republican on Monday.

"The president has failed to explain up to this point what follows the evident establishment of a no-fly zone over Libya, as it was originally described.

"Further, the president has articulated a wider political objective of regime change in Libya that is not the stated objective of our military intervention."

[Updated March 28, 8:08pm EST.]