The blast occurred at Ryongchon, 50km south of the North Korean border with China. And in a rare admission of need, North Korea on Friday asked for international help.
"They (North Korean authorities) have said that 150 people died in the explosion, including some schoolchildren... and over 1000 people were injured," said Ann O'Mahony, the regional director of Concern.
The International Federation of Red Cross and Red Crescent Societies suggested the toll could rise as rescue workers search through the rubble of 1850 houses that were levelled by the blast.
Another 6350 homes have been partially destroyed.
The country's request for help, which did not go into detail, was sent to the United Nations' main emergency relief organisation, the New York-based United Nations Office for the Coordination of Humanitarian Affairs (OCHA).
Talking to Irish state radio RTE, O'Mahony also revealed more details about the cause of the disaster.
"What they (the government) have said is that two carriages of a train carrying dynamite...they were trying to disconnect the carriages and link them up to another train... they got caught in the overhead electric wiring," she said.
"The dynamite exploded and that was the cause of the explosion," she said.
North Korea's Foreign Ministry said a train explosion was set off by gunpowder intended for use in an irrigation canal, Russia's Itar-Tass news agency reported from Pyongyang.
North Korea on Friday asked for international help, in a rare admission of need. The request, which did not go into detail, was sent to the United Nations' main emergency relief organisation, the New York-based United Nations Office for the Coordination of Humanitarian Affairs (OCHA).
Earlier news reports had spoken of a train collision involving carriages laden with fuel and explosives, possibly causing thousands of casualties.
The South Korean newspaper Joong Ang Ilbo reported that a cross-border train carrying a large number of Chinese was parked at the station when the two trains collided.
South Korean TV telecast give
details of the crash
The blast was so powerful it destroyed the railway station at around 1:00 pm (04:00 GMT) - just nine hours after North Korean leader Kim Jong-Il reportedly passed through it on his return from a trip to China.
North Korea's state-run news agency on Thursday confirmed that Kim had made a secretive trip to China on Monday through to Wednesday, but carried no comments on the reported explosion.
The country's official media were silent on the blast and the government immediately cut off international phone services to the devastated area in an effort to impose a news blackout.
China on Friday confirmed the first fatalities saying two Chinese were killed and 12 others injured in the disaster. It was the country's first official word of casualties in the incident.
The blast knocked down more than 20 houses, the Chinese Foreign Ministry and Xinhua said. Among the 12 injured Chinese, two sustained serious injuries and the remainder were lightly wounded, Xinhua said.
There was no sign in Dandong, the Chinese border city nearest to the site of Thursday's disaster, of injured people being brought out of North Korea.
"The area around Ryongchon station has turned into ruins as if it were bombarded"
South Korea's Yonhap news agency
Earlier, Seoul's MBC television said many of the injured were taken to hospitals across the border in the Chinese city of Dandong.
"They have already asked for help, but we can't give more details. We must report it to the provincial government," a Chinese official with the Dandong city government said.
"The area around Ryongchon station has turned into ruins as if it were bombarded," South Korea's Yonhap news agency quoted witnesses as saying.
"Debris from the explosion soared high into the sky and drifted to Sinuju," a North Korean town on the border with China, it said.
Yang Jong-hwa, a spokeswoman of South Korea's Unification Ministry, said her organisation could not immediately confirm the reports. The ministry is in charge of relations with North Korea.
The Defence Ministry likewise was not commenting.
"We are aware of the news reports, but we will not make any comments at this stage," said a spokesman, speaking on customary condition of anonymity.
North Korea declared a state of emergency around the site of the blast which resembled a war zone, South Korea's Yonhap news agency quoted Chinese sources as saying on Thursday.