Beijing  - Chinese troops on Thursday stepped up their efforts to rescue survivors of Monday's devastating earthquake and get vital aid to tens of thousands of people in isolated towns and villages in the south-western province of Sichuan.

Dozens of extra helicopters and planes were brought in to drop food, clothing and bedding to residents of the worst-hit counties of Beichuan and Wenchuan.

Premier Wen Jiabao, who flew over Wenchuan by helicopter on Wednesday, ordered an extra 90 military and civilian helicopters to be used in the relief operations, the official Xinhua news agency said.

An additional 30 transport planes were also set to begin dropping supplies on Thursday, the agency quoted officials as saying.

About 100,000 troops and paramilitary police were sent to earthquake-hit areas, where tens of thousands of people are believed to be buried in rubble.

Several towns close to the epicentre were nearly razed, a military relief officer said Wednesday, as the confirmed death toll rose to about 15,000.

At least 26,000 people were believed to be buried in collapsed buildings, while Xinhua said more than 30,000 people were missing or out of contact in Sichuan's Shifang city alone.

Rescue teams had treated more than 60,000 injured people by Thursday morning, the Sichuan disaster relief headquarters said.

Military helicopters began flying aid, medical personnel and relief troops to isolated settlements near the epicentre on Tuesday.

But the extent of the damage and casualties in Wenchuan, which has a population of 105,000, remained unclear.

A paramilitary officer who was one of the first outsiders to reach some of the worst affected areas on Tuesday said that several towns were almost razed to the ground.

Heavy rain had initially prevented helicopters from flying emergency aid to Wenchuan while troops were still restoring two badly damaged main roads to the county.

The 7.8-magnitude earthquake was felt in cities hundreds of kilometres away, including Beijing, Shanghai, Hong Kong and Bangkok. (dpa)