Tokyo ? One week after an earthquake and tsunami spawned a nuclear crisis, the Japanese government conceded Friday it was slow to respond to the disaster and welcomed ever-growing help from the United States in hopes of preventing a complete meltdown at the Fukushima Dai-ichi power plant.

The entire world was on alert, watching for any evidence of dangerous spikes in radioactivity spreading from the six-reactor facility, or that damage to the Japanese economy might send ripple effects around the globe.

As day broke today, steam rose from Unit 3, an unwelcome development if not a new one that signaled continuing problems. Emergency crews faced two continuing challenges at the plant: cooling the nuclear fuel in reactors where energy is generated and cooling the adjacent pools where thousands of used nuclear fuel rods are stored in water.

“In hindsight, we could have moved a little quicker in assessing the situation and coordinating all that information and provided it faster,” Chief Cabinet Secretary Yukio Edano said Friday.

Crucial to the effort to regain control over the plant is laying a new power line to the complex, allowing operators to restore cooling systems. Tokyo Electric said it has brought the cable to the plant and was expected today to try to connect it to the facility’s Unit 2; the utility has already missed a Thursday deadline to do that.

Power company official Teruaki Kobayashi warned that experts will have to check for anything volatile to avoid an explosion when the electricity is turned on. “There may be sparks, so I can’t deny the risk,” he said.

Even once the power is reconnected, it is not clear if the cooling systems will still work.

The storage pools need a constant source of cooling water. Even when removed from reactors, uranium rods are still extremely hot and must be cooled for months, possibly longer, to prevent them from heating up again and emitting radioactivity.

The government raised the raised the accident classification for the nuclear crisis from Level 4 to Level 5 on a seven-level international scale. That put it on a par with the Three Mile Island accident in Harrisburg, Pa., in 1979, and signified its consequences went beyond the local area.

Edano also said Tokyo was asking Washington for additional help, yet another change from a few days ago, when Japanese officials disagreed with American assessments of the severity of the problem.

The Science Ministry said radiation levels about 19 miles northwest of the Fukushima Dai-ichi plant rose at one time Friday to 0.15 millisieverts per hour, about the amount absorbed in a chest X-ray. While levels fluctuate, radiation at most points at that distance from the facility have been far below that. The ministry did not have an explanation for the rise.

A U.S. military fire truck was among a fleet of Japanese vehicles that sprayed water into Unit 3, according to air force Chief of Staff Shigeru Iwasaki, sending tons of water arcing over the facility in an attempt to prevent nuclear fuel from overheating and emitting dangerous levels of radiation.

Additionally, the United States also conducted overflights of the reactor site, strapping sophisticated pods onto aircraft to measure radiation aloft. Two tests conducted Thursday gave readings that U.S. Deputy Energy Secretary Daniel B. Poneman said reinforced the U.S. recommendation that people stay 50 miles away from the Fukushima plant.

American technical experts also are exchanging information with officials from the Tokyo Electric Power Co. which owns the plans, as well as with Japanese government agencies.

Sirens wailed along the devastated northeast coastline on Friday to mark one week since the prosperous country was stricken. Natural forces have claimed the lives of more than 6,900, with many thousands more missing in an area struck first by a magnitude 9.0 earthquake and then an enormous wall of water that seemed to scrape the earth clean.