WASHINGTON -- The United States and its allies shifted 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 Misrata forced Moammar Gadhafi'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 anti-aircraft fire sounded as warplanes streaked across the sky.

U.S. military officials said that the first stage of the military campaign, when more than 160 Tomahawk missiles fired from ships at sea largely destroyed Gadhafi'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 Gadhafi's forces that are attacking population centers," said Rear Adm. Gerard Hueber, the chief of staff for the U.S.-led operational command, speaking to reporters.

As long as Gadhafi'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.

Hueber also said that the coalition was communicating with rebel forces. But later, when he was pressed on whether the U.S. was telling rebels not to go down certain roads because there would be airstrikes there, he said he had misspoken.

President Barack Obama has said that the goal of the military assault is not to remove Gadhafi from power and that the United States will step back within days from playing the lead role in the attacks.

As the war intensified, Obama faced new pressures in Congress. The House speaker, John 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.