A series of pre-dawn explosions ripped through four residential blocks around the industrial city of Shijiazhuang in north China on Friday and Xinhua news agency said 18 people were killed.
The most powerful blast flattened a five-storey building housing workers in the Number 3 Cotton Mill. One local resident estimated 35 families lived in the block.
Local newspaper reporters said they were certain the death toll was much higher than reported by Xinhua.
There was no word on what caused the explosions in the capital of Hebei province 250 km (150 miles) southwest of Beijing.
Residents said they had no idea who might have set off the blasts, although one restaurateur said their size and precision indicated the work of professionals.
Last September, a series of blasts in Shijiazhuang department stores and on buses caused by crude time bombs injured 28 people, according to state media.
Responsibility was pinned on a single man who was also convicted by a local court of extortion and theft and was later executed, the media reports said without suggesting a motive.
Police threw a tight cordon around the whole city of 1.3 million, manning roadblocks, searching all vehicles and checking the identity papers of passengers. Several foreign reporters were turned back.
A wide area around the collapsed five-storey building was sealed off.
Shijiazhuang is a centre of China's declining cotton textile industry, a focus of Beijing's industrial restructuring that has led to millions of job losses around the country.