Blast in a road tunnel under construction in Chengdu, capital of southwest China's Sichuan Province.

One person went missing, and at least 20 others were injured after a blast occurred in a road tunnel under construction in Chengdu, capital of southwest China's Sichuan Province, Tuesday afternoon.

The blast assumed to have been caused by gas explosion happened at 1:20 p.m. in the No. 1 Tunnel of Wuluo Road.

All of the injured have been rushed to hospital. Two of them are still in critical condition. An investigation is under way