Japan is to raise the nuclear alert level at the Fukushima Daiichi power plant to a maximum seven, putting the emergency on a par with the 1986 Chernobyl disaster.

Nuclear safety officials had insisted they had no plans to raise the severity of the crisis from five – the same level as the Three Mile Island accident in 1979 – according to the international nuclear and radiological event scale.

But the government came under pressure to raise the level at the plant after Japan's nuclear safety commission estimated the amount of radioactive material released from its stricken reactors reached 10,000 terabecquerels per hour for several hours following the earthquake and tsunami that devastated the country's northeast coast on 11 March. That level of radiation constitutes a major accident, according to the INES scale.

The scale, devised by the international atomic energy agency, ranks nuclear and radiological accidents and incidents by their severity from one to seven.

Japan also temporarily issued tsunami warnings for parts of the north-east coast on Monday following another powerful aftershock. It is exactly a month since a magnitude-9 earthquake created huge waves that left an estimated 28,000 people dead or missing. NHK, the public broadcaster, warned of a tsunami up to 2 metres high on the coast of Ibaraki prefecture after the magnitude-7.1 quake.

Although the waves were estimated to be much smaller than those that hit on 11 March, the meteorological agency warned people in Ibaraki to evacuate to higher ground. The warnings were later lifted.

The aftershock came as the government said it was widening the evacuation zone around the plant due to high levels of accumulated radiation and fears about long-term effects on residents' health. A fire that broke out at the plant's number four reactor at 6.38am local time was extinguished, the operator, Tokyo Electric Power company (Tepco), said.

More than 14,000 people are still missing following the disaster, and 152,000 survivors are living in evacuation centres.

The prime minister, Naoto Kan, placed a message in newspapers in several countries, including Britain, China and the United States, thanking the international community for its support. Kan said the generosity shown towards Japan in its time of need demonstrated the human capacity for kizuna, or bonds of friendship, and vowed that Japan would emerge a stronger nation.

"We deeply appreciate the kizuna our friends from around the world have shown and I want to thank every nation, entity, and you personally, from the bottom of my heart," he said.

The government's chief spokesman, Yukio Edano, said the current 12-mile (20km) evacuation zone would be extended to five other communities, including the village of Iitate, which lies 25 miles from the plant.