The announcement by the State Council, or cabinet, was the clearest sign yet that the crisis at a quake-ravaged nuclear complex in northeast Japan could affect China's ambitious nuclear expansion, by far the world's largest.
But at least one expert said the measures were unlikely to stop China's expansion of nuclear power.
A State Council meeting chaired by Premier Wen Jiabao told Chinese residents they had nothing to fear about radiation drifting from Japan's Fukushima Daiichi plant.
But China's own nuclear power plans would face tougher scrutiny, said the account of the meeting on the government's website www.gov.cn.
"We will temporarily suspend approval of nuclear power projects, including those in the preliminary stages of development," the statement said.
The State Council called for use of "the most advanced standards" to proceed with a safety assessment of all nuclear plants under construction.
"Any hazards must be thoroughly dealt with, and those that do not conform to safety standards must immediately cease construction," the statement said.
The safety push could slow, but not stop, the expansion of nuclear power, which the government hopes will play a big role in plans to cut dependence on coal over the next decade, said one expert.
"The suspension of new project approvals is just a temporary one and will not influence China's long-term nuclear power construction plans," said Lin Boqiang, director of the Center for Chinese Energy Economics Research at Xiamen University.
"This is clearly the right thing to do and it is what every country will be doing to ensure that ordinary people are reassured about the safety of nuclear power plants."
The State Council said it had detected no abnormal levels of radiation in China from Japan. Chinese experts had concluded that wind would scatter any radiation from the crippled Japanese plant over the Pacific Ocean, said the statement.
Local governments across China have been vying for the investment, jobs and kudos that the new reactors would bring. They usually ally themselves with major nuclear operation companies to push projects, which require a series of permits and safety reports.
China is building about 28 reactors, or roughly 40 percent of the world's total under construction, and the central government has fast-tracked approvals in the past two years.
China now has only 10.8 gigawatts of nuclear generating capacity in operation after over two decades of construction, so the plan to get almost four times as much underway in the next five years marks a dramatic acceleration.
China's official target, drawn up in 2007, was to increase capacity to 40 GW by 2020.
But China's biggest reactor builders, the China National Nuclear Corporation and the China Guangdong Nuclear Power Corporation, have both said that the country could easily boost capacity to more than 100 GW. Analysts expect a revised target of 70-80 GW to be included in the country's new five-year plan for the energy sector due at the end of this month.
The main beneficiaries of the nuclear program included France's Areva and U.S.-based Westinghouse, a unit of Japan's Toshiba. Both have launched competing third-generation reactors in China.
But there are already signs that Japan's crisis has ignited public opposition in parts of China vying for nuclear plants.
Plans for a plant in Sichuan, the southwest Chinese province devastated by an earthquake in 2008, have attracted a surge of online denunciations in recent days.
The government of Nanchong, an area of Sichuan with more than 7 million residents, has been pressing to join China's nuclear charge.
Nanchong was on the outer edge of the earthquake that flattened entire towns in Sichuan in May 2008, and suffered only about 30 deaths out of the total 80,000 or so who perished then.
"This nuclear crisis in Japan has made me sense the dread of nuclear radiation," said one of many recent blog and Internet message-board comments about the planned Nanchong plant. "I vehemently oppose building nuclear power in Sichuan."