Two people were killed when a subway tunnel under construction collapsed in Xi'an, capital city of northwest China's Shaanxi Province on Sunday, news website Cnwest.com reported.

A photo taken on Sunday, August 2, 2009, shows the site of the collapsed subway tunnel under construction in Xi'an, capital of northwest China's Shaanxi Province. Two people were killed in the accident. [Photo:Cnwest]

The accident occurred at 9:20 a.m. at the Sajinqiao section of subway line one, where a 20-meter-wide area of the construction site gave way, trapping workers below. The workers were rescued shortly after the collapse but later died.

Authorities are investigating the incident, whose cause is still unknown.

The Xi'an Subway Construction Office, the department overseeing the project, has called for safety checks at all of its subway construction sites in the city.