By Ben Blanchard

DUJIANGYAN, China, May 13 (Reuters) - Huddled inside buses and under makeshift tents, survivors of China's earthquake sought shelter from the rain and cold on Tuesday as aid agencies warned that the most urgent risk they faced was exposure.

Nearly 10,000 people were killed by the 7.9 magnitude earthquake that hammered China's southwestern province of Sichuan. The epicentre was in Wenchuan, a hilly county about 100 km (62 miles) from the provincial capital, Chengdu.

"The disaster is in mountainous areas. In the mornings and evenings, it can get very cold or just a few degrees (Celsius), so shelter is very important," said Rhea Leung of the International Federation of Red Cross and Red Crescent Societies in Hong Kong.

"We are most concerned about shelter ... China Red Cross has supplied tents and blankets to some of the disaster areas," Leung said, as she appealed for more donations.

Some 1,300 rescue and relief troops arrived for the first time at Wenchuan county, the epicentre, and started operations in the rain, Xinhua news agency said.

In Dujiangyan, a city midway between Chengdu and the epicentre, mangled bodies could be seen lying on the streets covered in blue tarpaulin.

Doctors at a small hospital were treating patients outside the building. Structures that have not collapsed were evacuated as residents remained on the streets, fearing aftershocks.

A witness said it had been raining all day.

"It's around 17 degrees (Celsius, about 63 fahrenheit), it's very wet, people are using tents provided by the government. But most are using some ad hoc stuff (for shelter)," a Reuters correspondent said.

"People are sheltering in buses. They are not allowed in buildings because many are damaged. People are on the streets, which look like refugee camps. Some are trying to leave."

Another hospital, which had a wing destroyed, did not appear to be functioning.

"We can't do anything any more about the immediate deaths from the quake. Exposure is the most serious problem now because it can get really cold at night," said Lo Winglok, a leading medical doctor in Hong Kong.

"People need immediate shelter. Then we need to see how badly damaged are water supplies, as that could bring on diarrhoea."

Aid agencies such as Medecins Sans Frontier (MSF) and the IFRC have sent teams to Sichuan to assess what items were most urgently needed, but access was proving to be difficult.

"We have sent a team of two doctors and a logistics expert but we don't know if they have arrived in Sichuan as communications and transportation have been disrupted," said MSF's Gloria Hoo in Hong Kong. (Writing by Tan Ee Lyn; Editing by Nick Macfie)

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