* Magnitude 6.6 quake strikes China's Sichuan province
* Epicentre near Ya'an city, felt in provincial capital
* Premier Li visits disaster area
China was hit by its worst earthquake in three years on Saturday that killed at least 157 people and injured more than 5,700, the Ministry of Civil Affairs said.
The magnitude 6.6 quake hit a remote mountainous area of southwestern China's Sichuan province at 8:02 a.m. (0002 GMT), close to where an earthquake killed almost 70,000 people in 2008.
The quake struck in Lushan county, near the city of Ya'an, at a depth of 12 km (7.5 miles), the U.S. Geological Survey said. It was felt in the provincial capital, Chengdu, and in neighbouring provinces, causing many people to rush out of buildings, according to social network posts.
Most of the deaths were concentrated in Lushan. Pictures on Chinese news sites showed toppled buildings and people in bloodied bandages being treated in tents outside the hospital. Water and electricity in the area were cut off by the quake.
Premier Li Keqiang flew into the disaster zone by helicopter to voice support for the rescue operation.
"The first 72 hours is the golden period for rescue," Li told officials, the Xinhua news agency reported. "We cannot delay by a minute."
"Under the strong leadership of the party and the government, as long as we unite as one, and conduct the rescue in a scientific way, then there will be the conditions and the ability to minimise the losses to the greatest degree and to overcome the disaster," Li said.
Xinhua said 6,000 troops were in the area to help with rescue efforts. State television CCTV said only emergency vehicles were being allowed into Ya'an, although Chengdu airport had reopened.
Rescuers in Lushan had pulled 91 survivors out of rubble, Xinhua said. In villages closest to the epicentre, almost all low-rise buildings had collapsed, footage on state television showed.
"We are very busy right now, there are about eight or nine injured people, the doctors are handling the cases," said a doctor at a Ya'an hospital who gave her family name as Liu.
The hospital was treating head and leg injuries, she said.
The International Federation of Red Cross and Red Crescent Societies said it was in discussions with the Red Cross Society of China on whether international support was needed.
The China Meteorological Association warned of the possibility of landslides in Lushan county on Saturday and Sunday.
A resident in Chengdu, 140 km (85 miles) from Ya'an city, told Xinhua he was on the 13th floor of a building when he felt the quake. The building shook for about 20 seconds and he saw tiles fall from nearby buildings.
Ya'an is a city of 1.5 million people and is considered one of the birthplaces of Chinese tea culture. It is also the home to one of China's main centres for protecting the giant panda.
"There are still shakes and tremors and our area is safe. The pandas are safe," said a spokesman for Ya'an's Bifengxia nature park which houses more than 100 pandas.
Shouts and screams were heard in the background while Reuters was on the telephone with the spokesman.
"There was just an aftershock, an aftershock, our office is safe," he said.
Sichuan is one of the four major natural gas-producing provinces in China, and its output accounts for about 14 percent of the nation's total.
Sinopec Group, Asia's largest oil refiner, said its huge Puguang gas field was unaffected.
The U.S. Geological Survey initially put the magnitude at 7, but later revised it down. The devastating May 2008 quake was magnitude 7.9.