TOKYO — Scrambling to corral a widening crisis, engineers linked a power cable to the crippled Fukushima Dai-ichi Nuclear Power Station early today as they struggled to restart systems designed to prevent overheating and keep radiation from escaping.

The Tokyo Electric Power Co., which runs the station 140 miles north of here, said it hoped to connect the electric cord to the cooling equipment inside the facility later today in an attempt to stabilize the reactors that were damaged by the powerful earthquake and tsunami that struck Japan eight days ago. They were planning to start with Reactor No. 2, which was seen spewing steam, perhaps containing radioactive particles, on Friday. Engineers apparently think that reactor is better suited to test whether the pumps needed to circulate cooling water will function than other reactors that are more severely damaged.

Officials cautioned that restoring electricity to the reactor would prove fruitless if the pumps were not working. In that case, a new cooling system would be needed, leading to more delays in an emergency that has bedeviled the power company and the government, and caused anxiety and frustration overseas.

Japan’s Nuclear and Industrial Safety Agency said the crisis now had wider consequences and raised its assessment of the accident’s severity to a Level 5 on a seven-level scale established by the International Atomic Energy Agency. Hidehiko Nishiyama, a senior official at the agency, said the assessment was retroactive to Tuesday and based on the fact that officials now assumed that more than 3 percent of the nuclear fuel at the plant had undergone meltdown.

The adjustment was an admission by Japanese officials that the problem was worse than it had stated.

“We could have moved more quickly in collecting information and assessing the situation,” said Yukio Edano, the chief Cabinet secretary.

Outside experts have said for days that this disaster, which has entered its second week, is worse than the one at Three Mile Island in 1979 — which the United States classified as a 5 on the international scale but which released far less radiation outside the plant than Fukushima Dai-ichi already has.

Even if the new electric cable succeeds in jump-starting the cooling equipment at Reactor No. 2, much work remains. Reactors No. 3 and No. 4 have sustained severe damage.