WASHINGTON — The United States and its allies shifted on Wednesday to ferocious airstrikes on Libyan ground forces, tanks and artillery, marking the second phase of a military campaign that drew the Pentagon deeper into the fight.

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.