Washington ? A barrage of U.S.-led airstrikes opened the door for Libyan rebels to retake the eastern city of Ajdabiya Saturday, handing President Barack Obama a tangible example of progress as he defends the military action to war-weary Americans.

The administration has been under pressure to better explain why the U.S. was embroiling itself in another Muslim conflict and to clarify what America’s continuing role will be as it begins to turn control of the week-old operation over to NATO.

Obama cited “significant success” in the war Saturday, and he and others defended the U.S. intervention as lawful and critical to save thousands of lives and stabilize a strategically vital region in the Middle East.

“The United States should not and cannot intervene every time there’s a crisis somewhere in the world,” Obama said in his weekly radio and Internet address Saturday. But with Libyan leader Moammar Gadhafi threatening “a bloodbath that could destabilize an entire region … it’s in our national interest to act. And it’s our responsibility. This is one of those times.”

And Massachusetts Democrat Sen. John Kerry, chairman of the Senate Foreign Relations Committee, said the events in the Middle East could be “the most important geostrategic shift since the fall of the Berlin Wall.”

Without military intervention by the U.S. and NATO, “the promise that the pro-democracy movement holds for transforming the Arab world could have been crushed,” he said in The Wall Street Journal.

The Pentagon said U.S.-led forces pounded Libyan ground troops and other targets along the Mediterranean coast and in Tripoli, and the contested cities of Misrata and Ajdabiya in strikes overnight, but they provided no details on what was hit. A Pentagon spokesman, Navy Capt. Darryn James, says there were no Tomahawk cruise missile strikes overnight.

All together, the Pentagon said the U.S. military launched nearly 100 strikes overnight, just slightly higher than a day ago.

“Every day, the pressure on Gadhafi and his regime is increasing,” Obama said in the Saturday address, which aired just after Libyan rebels retook Ajdabiya, celebrating in the streets.

Still, even after a week of U.S.-led air strikes, Pentagon officials say that forces loyal to Libyan leader Moammar Gadhafi are a potent threat to civilians. And they are looking at plans to expand the firepower and airborne surveillance systems in the military campaign, including using the Air Force’s AC-130 gunship armed with cannons that shoot from the side doors, as well as helicopters and drones.

Obama, who will speak to the nation Monday evening, has been roundly criticized by lawmakers for not seeking more Congressional input on the war.

Top State Department lawyer Harold Koh said Saturday that U.S. had “ample international legal authority” to intervene in Libya and all Congressional requirements were met.

He said the “nature, duration and scope” of the operation do “not rise to the level” of requiring anything more than has already been done in terms of U.S. law, he said.

“I wish I lived in a world in which intervention was unnecessary, I don’t,” he said. He added that, “sometimes non-intervention is failure” citing the Bosnian city of Srebrenica and Rwanda.

Former Libyan ambassador to the United States Ali Aujali called Libya a unique situation.

“If no action will be taken, we will have another massacre in Africa that will be remembered like Srebrenica and Rwanda,” he said. “It was the right action at the right time.”