As rebels in Libya continued to establish their control of Tripoli on Monday, senior NATO officials said they were startled by the speed with which Moammar Gaddafi’s defenses have collapsed. The North Atlantic Council planned to meet later this week to discuss what to do next and what role it might play in a post-Gaddafi Libya, a NATO official in Brussels said.

“We cannot drop our guard when we know Gaddafi has done things even in full retreat,” the official said Monday, speaking under NATO ground rules of anonymity.

NATO officials in both Brussels and Naples, where the Libyan operations have been run, said that their mission still was not over. They noted that Libyan army holdouts elsewhere in the country may not know about the situation in Tripoli and could still pose a threat.

A power vacuum in Tripoli could also lead to further violence over the next several days, the official said, since much of the population is armed, not just rebel and government fighters.

Over the last several days, NATO’s daily operational updates have been filled with accounts of airstrikes on targets in the vicinity of Tripoli. On Saturday alone, the alliance hit at least 22 targets around the city, an unusually high one-day number for the nearly-five-month-old operation.

NATO officials said rebel advances in Zawiyah, a key oil refinery town 27 miles west of Tripoli, had pushed government forces out into the open back toward the capital, and that Gaddafi’s army also had been forced to resume using heavy armament such as tanks that are easy targets for airstrikes. For much of the last several months, government troops had been using light pickup trucks with machine guns mounted to their beds, similar to the vehicles used by the rebels, as a way of blending in and avoiding airstrikes.

“We strike whatever heavy equipment we see,” the NATO official in Brussels said. “We can’t take the chance” that it would later be used against civilians, he said.

NATO Secretary General Anders Fogh Rasmussen said Monday that “NATO is ready to work with the Libyan people and with the Transitional National Council,” and he called on holdout government forces to “end their careers of violence.”