The No. 3 reactor at the Fukushima No. 1 nuclear power plant is seen in this still image taken from NHK news program on March 13, 2011. Tokyo Electric Power Co., the utility supplier, notified the government early Sunday morning that the No. 3 reactor at the No. 1 Fukushima plant had lost the ability to cool the reactor core. The reactor is now in the process of releasing radioactive steam, according to top government spokesman Yukio Edano.

TOKYO, March 13  -- Japan is facing an escalating nuclear threat on Sunday as the country's top government spokesman warned that radioactive meltdowns may have occurred in two reactors of the Fukushima nuclear power plant.

"We are acting on the assumption that there is a high possibility that one has occurred" in the No. 1 reactor of the quake-hit plant, Chief Cabinet Secretary Yukio Edano told a press conference on Sunday while being asked whether or not meltdowns had occurred.

"As for the No. 3 reactor, we are acting on the assumption that it is possible," he said.

The latest reports of Japan's Sankei Shimbun said that pressure has been successfully released at the No. 3 reactor of the Fukushima No. 1 nuclear plant following the injection of fresh water.

Tokyo Electric Power Co.(TEPOC), operator and owner of Fukushima nuclear plants, said early on Sunday that a sixth reactor at the nuclear power plants has lost its ability to cool the reactor core since Friday's quake.

The No. 3 reactor at Fukushima No. 1 nuclear power plant lost the cooling function after No. 1 and No. 2 reactors at the No. 1 plant and No. 1, No. 2 and No. 4 at the No. 2 plant had suffered the same trouble.

At an emergency press conference early Sunday, Edano said the reactor is releasing radioactive steam after the malfunction occurred.

An explosion occurred at the Fukushima No. 1 nuclear power plant Saturday, destroying the roof and the walls of the building of the No. 1 reactor's outer container.

Four people were injured at the power plant, but radiation levels dropped quickly after surging for a while following the blast.

The top government spokesman warned that 114 people are still staying within a 10-km radius of Fukushima No.1 nuclear power plants and 180,000 in the 20-km evacuation zone.

All inhabitants have been evacuated from a 3-km radius of the No. 2 plant, and authorities have begun evacuating more than 30,000 from a 10-km zone around the plant, he added.

On Saturday, authorities expanded the evacuation zone from a 10-km radius for the Fukushima nuclear plants to a 20-km radius.

Kyodo News reported Sunday that 19 more people were found to have been exposed to radiation, in addition to the three exposure cases recorded Saturday,

Tokyo Electric Power Co. has notified Japan's nuclear safety agency that the radiation level at the Fukushima No. 1 nuclear power plant has exceeded the legal limit.

The Japanese authorities on Sunday upgraded the magnitude of the earthquake which hit the country's northeastern and eastern regions Friday to 9.0 from 8.8, Kyodo News reported, citing the Japan Meteorological Agency.

So far, the number of people who have died or remained unaccounted for after the quake have exceeded 2,000, the police said. The official death toll was around 700, Kyodo News said.

In Fukushima Prefecture alone, 1,167 were unaccounted for and well over 600 corpses had been found in both Fukushima and Miyazaki prefectures, said the report.

There are also tens of thousands of people that local governments have been unable to contact, police and local officials said.

Japanese Defense Minister Toshimi Kitazawa said Sunday that Prime Minister Naoto Kan has instructed the defense ministry to increase the size of Self-Defense Forces for quake relief operations from 50,000 to 100,000 troops.

While the country continues to grapple with the widespread damage from the massive earthquake, countries across the world have also offered a helping hand to it.

A total of 50 nations and regions have promised to provide with relief support, and offers from over 70 specialist rescue services from around the globe have been received by Japan.

A 15-member Chinese rescue team arrived at Tokyo's Haneda Airport Sunday for quake relief operations.

The Chinese International Search and Rescue Team (CISAR) brought with them 4 tons of materials and equipment for search and rescue, power supply and telecommunication.

The rescuers will immediately head for quake-hit areas to search for survivors from Friday's great earthquake and ensuing tsunami.