China has declined Australia's offer to provide an expert search and rescue team to help find survivors of a massive earthquake, citing logistical problems.

Similar offers of help from Japan and South Korea have also been politely turned down.

"We were told that China cannot receive rescuers now due to poor condition of transportation systems," said a Japanese foreign ministry official in charge of emergency aid.

Beijing's response came a day after rescue workers made it on foot to the epicentre of Monday's earthquake to find some towns and cities "razed to the ground".

More than 20,000 people are believed to have died in the quake, but tens of thousands of people are understood to be buried in the rubble of their homes, schools and workplaces in the quake zone.

Prime Minister Kevin Rudd offered Australian aid to the Chinese after the 7.9- magnitude quake rocked Sichuan province.

"China responded today thanking Australia for the offer," a spokesman for Australia's Department of Foreign Affairs and Trade said.

"The extreme challenges of transport and communication in the earthquake region mean that at this point the aid cannot be received."

Mr Rudd, who speaks fluent Mandarin and who once worked as a diplomat in Beijing, said that he had offered assistance, including search and rescue capability, to Premier Wen Jiabao.

The Government's disaster coordination body Emergency Management Australia had been on standby to send search and rescue personnel on a commercial flight to the area worst-affected by the quake in the country's south-west.

While it has kept the door open to foreign offers of help, China says that conditions are "not yet ripe" for international rescue workers to enter the country, citing damage to transport links in affected areas.

"Razed to the ground"
Rescue workers who reached Wenchuan County, the epicentre of the devastating earthquake , say some towns were "razed to the ground", with not a single house left standing.

Several thousand army troops and medical workers walked into the area late on Tuesday, after damaged roads and bad weather made it impossible to drive or fly in.

China has now mobilised 100,000 military and police, with troops parachuting and speed boating into cut-off areas while planes and helicopters air-dropped emergency supplies.

"The losses have been severe. Some towns basically have no houses left. They have all been razed to the ground," Wang Yi, head of an armed police unit sent into the disaster zone, was quoted as saying by Sichuan Online news site.

He did not say how many of Wenchuan's eight main towns had suffered such a high level of damage, but said they included the towns of Yingxiu, Xuankou, and Wolong.

"Mountain villages in the surrounding area have also been basically razed to the ground," he added.

Earlier reports said that in the city of Mianyang alone, nearly 20,000 people were believed to be buried, while elsewhere in Beichuan County, photos showed only around a fifth of buildings were still standing.

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