China said on Wednesday the black boxes of a Boeing 737-800 jet that crashed last month were badly damaged, leaving virtually no publicly available clues to explain its violent plunge into a wooded hillside, killing all 132 people on board.

China Eastern flight MU5735 from Kunming to Guangzhou plunged from cruising altitude into the mountains of Guangxi on March 21 in China's first fatal air crash since 2010.

Summarizing its preliminary crash report, the Civil Aviation Administration of China (CAAC) did not include any information from the cockpit voice and data recorders, which had been sent to Washington for analysis.