The lake was formed when a huge landslide blocked the Jian river near the epicentre and is now held in by dozens of fragile dams that pose a major threat in the disaster zone.

Pressure is building behind the dams as rivers and streams feed into the newly formed lakes. Officials fear the loose soil and debris walls of the dams could easily crumble once the water level reaches the top and begins cascading over.

As soldiers worked to dig a giant sluice to ease pressure on the Tangjiashan lake, another strong aftershock hit the quake-ravaged region yesterday, with the tremor felt in the major city of Chengdu.

The aftershock struck late afternoon, just 41km from the already quake-affected city of Guanygyuan. The US Geological Survey said the aftershock had a magnitude of 5.2.