More than 40,000 people are thought to be dead, missing or buried under rubble after China's devastating earthquake.

As relief teams reach areas close to the epicentre, they have discovered whole towns all but wiped off the map, with a confirmed death toll of around 15,000.

Bad weather is hampering efforts to rescue survivors after what was China's biggest earthquake in three decades.

Officials have also warned of dangers from increased strain on local dams as well as mudslides on brittle hillsides where rain has been forecast over the next few days.

Two hydropower stations in Maoxian County, where 7,000 residents and tourists remain stranded near the epicentre, were 'seriously damaged'. Authorities warned that the dams could burst.

Landslides had blocked the flow of two rivers in northern Qingchuan county, forming a huge lake in a region where 1,000 have already died and 700 are buried, state news agency Xinhua said.

The quake had also stopped a river in the stricken Mianzhu region, prompting officials to evacuate residents and drain dams, downstream, the agency said.

The destruction around the epicentre in remote Wenchuan county is extensive, with whole mountainsides sheared off, highways destroyed and building after building levelled.

Xinhua has reported more than 5,400 dead in Mianyang, up to 5,000 dead in Beichuan, 3,000 in Mianzhu, 2,600 in Deyang, 500 so far in Wenchuan, hundreds more in the provincial capital Chengdu and other towns and cities.

Xinhua reported that at least 7,700 people died in the small town of Yingxiu alone.

Tens of thousands are missing, many of them believed to be buried under the rubble of collapsed buildings.

Planes and helicopters have flown dozens of sorties, dropping tonnes of food and relief aid into the worst-hit zones, most of it cut off from the outside world by landslides and road closures.

As well as in Yingxiu, air drops have also been made in nearby Mianyang, Mianzhu and Pengzhou, while helicopters flew to Wenchuan with food, drinks, tents, communications equipment and other supplies.

Chinese Premier Wen Jiabao said 100,000 military personnel and police had been mobilised, indicating the huge scale of the disaster.

Mr Wen rushed to the scene hours after the 7.9 magnitude tremor hit on Monday.

Meanwhile, the Hong Kong autonomous region has pledged HK$350m (€29m) in aid, becoming one of the biggest donors to the international relief effort.

Read Margaret Ward's account of the rescue effort at Juyuan Middle School in Dujiangyan here