WASHINGTON — Despite widespread praise in Western capitals for NATO’s leadership of the air campaign in Libya, a confidential NATO assessment paints a sobering portrait of the alliance’s ability to carry out such campaigns without significant support from the United States.

The report concluded that the allies struggled to share crucial target information, lacked specialized planners and analysts, and overly relied on the United States for reconnaissance and refueling aircraft.

The findings undercut the idea that the intervention was a model operation and that NATO could effectively carry out a more complicated campaign in Syria without relying disproportionately on the United States military. Even with the American help in Libya, NATO had only about 40 percent of the aircraft needed to intercept electronic communications, a shortage that hindered the operation’s effectiveness, the report said.

Mounting an operation in Syria would pose a bigger challenge than the seven-month campaign that drove Col. Muammar el-Qaddafi of Libya from power, American officials said. Syria has a more capable military as well as a formidable array of sophisticated Russian-made air defenses that Pentagon officials say would take weeks of airstrikes to destroy.