BEIJING — The collapse of an under-construction railway bridge over a major river in China has killed at least 12 workers and left four others missing, state media reports said.

Aerial photos from the official Xinhua News Agency show a large section missing from the bridge’s curved aquamarine arch. A bent section of the bridge deck hangs downward into the Yellow River below.

Sixteen workers were on the bridge in northwest China’s Qinghai province when a steel cable snapped about 3 a.m. Friday during a tensioning operation, Xinhua said. Boats, a helicopter and robots were being used in the search for the missing.

The bridge is 1.6 kilometers (1 mile) long and its deck is 55 meters (180 feet) above the surface of the river below, the English-language China Daily newspaper said.