ZAO, Japan --Smoke billowed from a building at Japan's crippled nuclear power plant Friday as emergency crews worked to reconnect electricity to cooling systems and spray more water on the overheating reactors at the tsunami-ravaged facility. Four of the troubled Fukushima Dai-ichi plant's six reactors have seen fires, explosions or partial meltdowns in the week since the tsunami. While the reactor cores where energy is generated are a concern, Japanese and U.S. officials believe a critical danger are the pools used to store spent nuclear fuel: fuel rods in one pool were believed to be at least partially exposed and in danger of leaking radiation. Friday's smoke came from Unit 2, and its cause was not known, the nuclear safety agency said. An explosion had hit the building on Tuesday, possibly damaging a crucial cooling chamber that sits below the reactor core. More urgent, Japan's chief government spokesman said, was the adjacent Unit 3. Fuel rods there may have been partially exposed, and without enough water, the rods may heat further and possibly spew radiation. Frantic efforts were made Thursday to douse the unit with water, using helicopters and firetrucks, and authorities prepared to repeat the effort Friday. "Dealing with Unit 3 is our utmost priority," Chief Cabinet Secretary Yukio Edano told reporters. In the week since the massive earthquake and tsunami, Japan's government and the utility that runs Fukushima have struggled to contain the plant's cascading troubles. Edano said Friday that Tokyo is asking the U.S. government for help and the two are discussing the specifics. "We are coordinating with the U.S. government as to what the U.S. can provide and what people really need," Edano said. The U.S. and Japan, close allies, have offered differing assessments over the dangers at Fukushima in recent days. U.S. Nuclear Regulatory Commission Chairman Gregory Jazcko said in Washington Thursday that it could take days and "possibly weeks" to get the complex under control. He defended the U.S. decision to recommend a 50-mile (80-kilometer) evacuation zone for its citizens, wider than the 30-mile (50-kilometer) band Japan has ordered. Crucial to the effort to regain control over the Fukushima plant is laying a new power line to the plant, allowing operators to restore cooling systems to the reactors. The operator, Tokyo Electric Power Co., missed a deadline late Thursday but said Friday workers hoped to complete the effort, first reconnecting Unit 1. Also Friday, the Group of Seven major industrialized countries agreed to support Japan -- whose infrastructure and industries were badly battered by the disasters -- by intervening in currency markets. The group did not say what it would do but the efforts would likely focus on weakening the Japanese yen, which has risen this week. A strong yen could make Japanese exports less competitive, crimping any recovery. The quake and unfolding nuclear crisis have led to power shortages in Japan, forced auto and other factories to close, sending shockwaves through global manufacturing and trade, and triggered a plunge in Japanese stock prices.