China National Petroleum Corp (CNPC) said the threat of a swollen "quake lake" which could overflow in the nation's southwest on Friday had put the country's longest oil pipeline at risk.

The Lanzhou-Chengdu-Chongqing pipeline is 60 kilometres (37.5 miles) downstream from the landslide-created Tangjiashan lake in Sichuan province, CNPC said in a statement on its website.

CNPC said the pipeline was designed to withstand a flood only five times weaker than the one that could come streaming through the region if the lake burst its banks.

As the only refined oil pipeline to southwestern China, it supplies 70 percent of oil-products used in Sichuan and the neighbouring municipality of Chongqing, the statement posted late on Thursday said.

The pipeline starts in Lanzhou, the capital of northwestern Gansu province, then passes through Shaanxi and Sichuan provinces before reaching Chongqing.

The pipeline has the capacity to transport more than six million tonnes of oil products a year.

Oil supplies to disaster-hit zone and neighbouring provinces risk being cut off if the pipeline is damaged and cannot be repaired within three days, CNPC said.

More than 600 soldiers were standing by to repair any damage to the pipeline, it said.

The Tangjiashan lake is one of 34 quake lakes formed by massive landslides triggered by the 8.0 magnitude earthquake that hit Sichuan province in May 12. Authorities have warned it could burst its banks at any time. More than 250,000 people have already been evacuated from downstream areas.