Seven people died and 23 others were injured, including six in critical condition, after a bus overturned inside an expressway tunnel in northwest China's Shaanxi Province, local traffic police said Tuesday.

The accident happened at about 11 a.m. Monday on the Shaanxi section of the Fuzhou-Yinchuan expressway, a spokesman with the Shanyang County police said.

The bus, carrying 35 people including the driver, was heading for Yunxian County in central China's Hubei Province, survivors from the accident said.

The bus suddenly lost control inside the tunnel and crashed onto the wall and guardrails before it overturned on the middle of the road, according to the survivors.

An initial probe indicated that speeding was to blame for the accident, police said.

Further investigation into the accident is under way.