The government of Japan, already battling against the deadly ravages of Friday’s earthquake-tsunami, has warned that the troubled No. 2 reactor at the badly-damaged Fukushima No. 1 nuclear power plant was still unstable, almost nearly five hours after workers poured seawater into it to try to cool down the fuel rods.

Chief Cabinet Secretary Yukio Edano said the reactor ‘‘is not necessarily in a stable condition,’’ at a press conference Tuesday morning (Tokyo time).

Earlier, the plant’s operator, the Tokyo Electric Power Co. (TEPCO) said pressure inside the reactor’s pressure container had fallen and that it believed seawater was successfully injected in. However, TEPCO cautioned that a rise in water levels had not been confirmed.

As radiation levels climb around the nuclear plant, TEPCO believes the core of the No. 2 reactor may have partially melted.

The No. 2 reactor automatically turned after the earthquake struck on Friday. Its cooling function ceased on Monday, while water levels rapidly declined, thereby fully exposing fuel rods for between two and three hours.

TEPCO further said that pressure inside the container that holds the rods rose after the container’s steam vents closed for unknown reasons, preventing seawater injection.

Some steam valves were later opened, TEPCO said, and the injection of seawater was resumed.

Already, two hydrogen explosions were reported at the site’s two other reactors over the weekend and Monday.

“A worrisome situation remains but I hope we can overcome this crisis,’’ said Prime Minister Naoto Kan, who is organizing a committee of government ministers and TEPCO officials to study the situation. ‘‘I will take all measures so that damage will not expand.’‘

Edano said he think the problem at the plant will not develop into a situation similar to the [1986 accident at the nuclear power reactor in] Chernobyl in the Soviet Union, even in the worst case.

The Nuclear and Industrial Safety Agency also said the worst case scenario will be less destructive than the Chernobyl incident, since TEPCO has depressurized the reactors by releasing radioactive steam.