WASHINGTON – Defense Secretary Robert M. Gates stood on the steps of the Pentagon late Tuesday alongside Liam Fox, his counterpart from Britain, America’s closest ally. Questions swirled here, in Europe and across North Africa whether NATO was specifically trying to find and kill Col. Muammar el-Qaddafi, the Libyan leader, with airstrikes.

Mr. Gates patiently repeated the alliance’s longstanding policy that it was attacking only legitimate military targets in Libya in order to degrade the ability of the government’s forces to threaten its civilian population. There was no targeted assassination effort under way.

“We have considered all along command-and-control centers to be a legitimate target, and we have taken those out elsewhere,” Mr. Gates said.

“Those centers are the ones that are commanding the forces that are committing some of these violations of humanitarian rights, such as in Misurata,” he added. “We are not targeting him specifically, but we do consider command-and-control targets legitimate targets wherever we find them.”

That careful, diplomatic language from the defense secretary came as administration officials and NATO officers in Europe confirmed that the alliance plans to step up attacks on the palaces, headquarters and communications centers that Colonel Qaddafi uses to maintain his grip on power.

As government forces and Libyan rebels face off across front lines that are becoming somewhat static, if not stalemated, NATO officials acknowledge that they have been frustrated by the resilience of Libya’s military — and by Colonel Qaddafi’s ability to hold on.

“I am impressed by his military infrastructure,” said one NATO officer involved in the air campaign. “Ammo compounds. Sophisticated communications infrastructure. We have slowed down the fighting at the front, so we are starting to focus our attacks on his supply lines, his ammo depots, his transport trucks, his fuel trucks.”

And his palaces, headquarters and communications centers, even in the capital, Tripoli.

“Now we are going after his rear echelon,” the NATO officer said. “We are going after his ability to command and control his forces — his headquarters, his command posts, his communications – all those things that allow him to coordinate his attacks at the front.”