Work has been halted again at Japan's Fukushima Daiichi plant after black smoke was seen rising from reactor 3.
Engineers were earlier forced to halt testing of the electrical system at reactor 2 as radiation levels spiked. There is also concern about the rising temperature at reactor number 1.
Power cables have been connected to all six reactors, and lighting has been restored at reactor 3. The plant's operator, Tokyo Electric Power (Tepco), had hoped to try to power up water pumps to reactor 3 on Wednesday but it is unclear whether that will now happen.
Tepco has said restoring power to all the reactor units could take weeks or even months. Engineers' efforts have been frequently hampered by smoke and spikes in radiation.
On Tuesday, an International Atomic Energy Agency (IAEA) senior official, James Lyons, said he could not confirm that the damaged reactors were "totally intact" or if they were cracked and leaking radiation.
"We continue to see radiation coming from the site... and the question is where exactly is that coming from," Mr Lyons told a news conference.
Meanwhile, Tokyo's tap water is unfit for babies to drink after radiation from Japan's quake-hit nuclear plant affected the capital's water supply, officials said.
Radioactive iodine levels in some areas were twice the recommended safe level, according to the BBC. People in Fukushima prefecture, where the nuclear plant is located, have been told not to eat certain vegetables because of contamination worries.
The authorities are warning people living in Tokyo not to allow babies less than a year old to drink water from the tap.
The level of radiation picked up in tests carried out on Tuesday was more than twice the level that is safe for infants to drink. Officials have stressed that children would have to drink a lot of it before it harmed them. There is no immediate health risk to others.
Reactor 1: Damage to the core from cooling problems. Building holed by gas explosion. Power lines attached.
Reactor 2: Damage to the core from cooling problems. Building holed by gas blast; containment damage suspected. Power lines attached.
Reactor 3: Damage to the core from cooling problems. Building holed by gas blast; containment damage possible. Spent fuel pond partly refilled with water after running low. Power lines attached.
Reactor 4: Reactor shut down prior to earthquake. Fires and explosion in spent fuel pond; water level partly restored. Power lines attached.
Reactors 5 & 6: Reactors shut down. Temperature of spent fuel pools now lowered after rising high. Power lines attached.
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