Fires, explosions and a partial melting of cores have taken place at four of the six reactors at the Fukushima nuclear center.

Officials say the situation in the number three reactor is the most serious. Mixed oxide fuel rods are partly made of plutonium. These fuel rods are partly exposed. Without water, they will continue to heat up and possibly release a large amount of radiation.

On Friday, the government raised the severity rating of its nuclear disaster from four to five on a seven-point international nuclear event scale. Level five means an accident with wider results, including some release of radioactive material. It also means a high possibility of the material making contact with people.

In Washington on Thursday, President Obama said the United States will stand with Japan as it contains its nuclear crisis, recovers and rebuilds.

PRESIDENT OBAMA: “The Japanese people are not alone in this time of great trial and sorrow. Across the Pacific, they will find a hand of support extended from the United States as they get back on their feet.”

A baby arriving from Japan receives nuclear radiation detection from at Kuala Lumpur International Airport in Sepang, Malaysia.

The president noted some of the steps the United States has taken to help.

PRESIDENT OBAMA: “Search and rescue teams are on the ground in Japan to help the recovery effort. A disaster assistance and response team is working to confront the aftermath of the earthquake and tsunami. The U.S. military, which has helped to ensure the security of Japan for decades, is working around the clock.”