A pounding from allied warplanes in the rebel-held city of Misurata forced Col. Muammar el-Qaddafi’s troops to pull back for much of the day, residents said, but by nightfall his forces had renewed their attacks. Government tanks terrorized the city, in one instance firing a shell that landed 20 yards from a hospital door. In Tripoli, small bursts of antiaircraft fire sounded as warplanes streaked across the sky.  American military officials said that the first stage of the military campaign, when more than 160 Tomahawk missiles fired from ships at sea largely destroyed Colonel Qaddafi’s air defenses and air force, had made the skies safe for coalition warplanes. The allies were conducting stepped up attacks on ground troops, military officials said, without fear of being shot down.  “We are interdicting and putting the pressure on Qaddafi’s forces that are attacking population centers,” said Rear Adm. Gerard P. Hueber, the chief of staff for the American-led operational command, speaking to reporters by audio link from a ship in the Mediterranean.  Admiral Hueber said the United States and its allies were striking at Colonel Qaddafi’s ground forces in both Misurata in the west and Ajdabiya in the east. Air attacks in such urban areas, which have the potential for many civilian casualties, meant the military was operating in “an extremely complex and difficult environment,” he said.  The goal was to “interdict those forces before they enter the city, cut off their lines of communication and cut off their command and control,” the admiral said.  As long as Colonel Qaddafi’s forces were fighting in and around cities where the allies had ordered them to back off, he said, coalition attacks would continue. He said the allies are in communication with the Libyan units about what they need to do, where to go and how to arrange their forces to avoid attack, but there was “no indication” that the government’s ground forces were following the instructions.  Admiral Hueber also said that the coalition was communicating with rebel forces. But later, when he was pressed on whether the United States was telling rebels not to go down certain roads because there would be airstrikes there, he said he had misspoken. American military officials have said there are no “official communications” with the rebels, which remains a delicate issue. Contact with the rebels would reflect a direct American military intervention in the civil war of another country.  President Obama, who returned to Washington on Wednesday from a trip to Latin America, has said that the goal of the military assault is not to remove Colonel Qaddafi from power and that the United States will step back within days from playing the lead role in the attacks.  But by striking directly at Libyan fighting forces in the midst of battle against the rebels and seeking to protect civilians, the United States and its allies highlighted the thin line between all-out war and the more limited mission, set out by a United Nations resolution, of enforcing a no-fly zone and forcing the pro-Qaddafi fighters to withdraw.  As the war intensified, Mr. Obama faced new pressures in Congress. The House speaker, John A. Boehner, sent the White House a letter on Wednesday, demanding answers about the cost of the war, an exit strategy and when the United States would hand off the lead role to the allies. He also asked a question central in Washington: “Is it an acceptable outcome for Qaddafi to remain in power after the military effort concludes in Libya?”  Obama administration officials continued to say that although they were not specifically seeking to attack Colonel Qaddafi, the Libyan leader might be ousted from power by his own family or inner circle. “I think there are any number of possible outcomes here, and no one is in a position to predict them,” Defense Secretary Robert M. Gates, said during an official visit to Cairo.  Both Mr. Gates and Secretary of State Hillary Rodham Clinton cited defections within Colonel Qaddafi’s ruling circle and alluded to divisions within his family, although they provided no details.  “We’ve heard about other people close to him reaching out to people that they know around the world — Africa, the Middle East, Europe, North America, beyond, saying: ‘What do we do? How do we get out of this? What happens next?’ ” Mrs. Clinton told Diane Sawyer of ABC News on Tuesday night.  On Wednesday, Mrs. Clinton cited reports of others now considering defecting, though she did not elaborate.  “We’ve been hearing a lot of things from many different sources,” she said at the State Department after a meeting with the foreign minister of Morocco, Taieb Fassi Fihri. “But what is very clear is that Qaddafi has lost his legitimacy to govern and the confidence of his people.”  Mrs. Clinton defended the operation so far. “I know that the nightly news cannot cover a humanitarian crisis that thankfully did not happen,” she said, “but it is important to remember that many, many Libyans are safer today because the international community took action.”  NATO inched closer on Wednesday to agreeing to who would be in charge of military operations once the United States stepped back, although strains remained.  The French foreign minister, Alain Juppé, said that representatives from the United States, Europe, Africa and the Arab nations would meet next week in London to discuss who would take the lead. But Germany, which has opposed military intervention in the Libya crisis, said it was withdrawing four of its ships in the Mediterranean from NATO command. To offset the impact of its action on other NATO allies, Germany said it would send 300 more troops to Afghanistan to help operate surveillance aircraft, German officials said.  In Tripoli, there was not nearly as much antiaircraft fire on Wednesday as on previous nights, suggesting that allied airstrikes may have taken out some of the larger guns or heavily armed positions.  In Ajdabiya, which has changed hands from the rebels to Qaddafi loyalists several times, residents said relentless shelling by loyalist troops had forced them to flee. One report called the city a “ghost town.”  Elisabeth Bumiller reported from Washington, and David D. Kirkpatrick from Tripoli, Libya. Reporting was contributed by Thom Shanker from Cairo; Steven Erlanger and Alan Cowell from Paris; and Mark Landler and Steven Lee Myers from Washington.
The United States and its allies shifted to ferocious airstrikes on ground forces, tanks and artillery, marking the second phase of a military campaign that drew the Pentagon deeper into the fight.