ISLAMABAD, Pakistan — Prime Minister Nawaz Sharif on Tuesday ceremonially broke ground on a $9.59 billion nuclear power complex to be built in Karachi with China’s help, seeking to ease Pakistan’s long-running energy crisis and signaling a new step by China in becoming a top nuclear supplier.

The deal, which officials said was still being finalized, is a major new advance in energy cooperation between the two countries, dwarfing previous reactor projects built along with China at Chashma, in Pakistan’s interior. And it establishes a growing counterpoint to a nuclear axis between the United States and India in recent years that Pakistani officials have seen as an irritant and Chinese officials have seen as a geopolitical challenge.

“The beginning of the 2,200-megawatt power project is indeed a proud moment in the energy history of Pakistan,” Mr. Sharif said at the ceremony, adding that the construction was to be completed in six years. The Chinese ambassador to Pakistan, Sun Weidong, and officials from the Pakistan Atomic Energy Commission attended.

Energy supply remains one of the most pressing concerns in Pakistan, and it was a major issue in the general elections last May, when Mr. Sharif promised to end crippling shortages. After taking office, his first visit was to China, where he discussed the nuclear power plant project.