China struggled to keep roads open to provide a lifeline for quake survivors, while the government warned Wednesday that rebuilding after the disaster would be "arduous."

The magnitude 7.9 quake that struck May 12 sent dirt and rocks tumbling into valleys, blocking roads to hinder relief efforts and clogging rivers that have developed into fast-rising lakes.

"We are racing against time to repair damaged infrastructure," said Mu Hong, a deputy director at the National Development and Reform Commission, the country's top economic planning body, adding that some roads were only reopened on a temporary basis.

"The high risk of mudslides and landslides makes our efforts more difficult," he said.

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In the disaster zone, 158,000 people have been evacuated and dozens of villages emptied in case the newly formed Tangjiashan lake bursts before soldiers and engineers can drain it, state media said Wednesday.

At the riverside Tongkou village downstream from the lake, residents had been moved to a camp up the hill but were returning each day to tend to their fields.

"If the water comes down from the burst dam, somebody will launch a fireworks signal to give us warning so everybody can run uphill," said villager Wang Hongyun. "Without seeing the warning, we will keep on gathering our crops."

Troops used explosives to clear debris and helicopters to airlift heavy moving equipment to dig drainage channels from the lake, located about 2 miles above the devastated town of Beichuan.

Forty machines, including excavators, are at the site, which is unreachable by road, and hundreds of troops are working around the clock to build the channel, the official Xinhua News Agency reported.

Yang Hailiang, the official heading the operation, said the teams were making good progress thanks to clear weather, and that they were one-third of the way through the job, Xinhua said.

Premier Wen Jiabao told a meeting of the State Council, China's Cabinet, that handling the danger from the swelling lakes was the "most pressing task" in the disaster recovery effort, the newspaper said.

The government has allocated $28.6 million to deal with the swelling lakes, Xinhua said. Of 34 lakes created by the earthquake in the mountainous province, 28 were at risk of bursting, according to the agency.

Government officials said earlier that the recovery effort would take three years in hard-hit Sichuan province.

"Due to the immense magnitude of loss resulted from the quake, production recovery and reconstruction of the quake-hit region will be arduous in the near future," the commission said in a statement.

Meanwhile, the number of confirmed deaths from the quake climbed toward an expected toll of more than 80,000. China's Cabinet said Wednesday that 68,109 people were killed, with 19,851 still missing.

In Japan, officials said China had asked Japanese soldiers to deliver earthquake relief aid in what would be the first significant military dispatch between the two countries since World War II.

Beijing has allowed foreign rescue and medical experts from several countries to help in the relief operation — a switch from China's usual approach of preferring to handle internal matters on its own.

"Our understanding is that the request is to fly a plane belonging to the Self Defense Forces to deliver its tents and blankets to a Chinese airport," Chief Cabinet Secretary Nobutaka Machimura said. "A decision shouldn't take too long, and we are working on it now."

Japan invaded China and set up a puppet regime in Manchuria in 1932, then conquered larger areas of the country before being defeated by the Allies in 1945. Many Chinese still resent Japan for its military aggression at that time.

Though postwar relations between China and Japan have been rocky, the two countries have grown closer in the past 18 months.