North Korea has broken its silence to confirm what it called a "very serious" train disaster at Ryongchon station near the Chinese border on Thursday.
Soldiers on the border with China appear unaware of the disaster
It is feared hundreds were killed and thousands hurt, but the state news agency gave no figures.
Reporting from the scene, China's state news agency said 154 people, including many children, were known to have died.
Aid workers and diplomats have gone to Ryongchon after a rare call for help from the secretive state.
A North Korean official, Jang Song-gun, who is in charge of rescue efforts, said the 154 confirmed dead included 76 schoolchildren, China's Xinhua news agency reported.
Xinhua also said 1,300 people had been injured by the blast which has left two huge holes some eight metres (26 feet) deep.
The Chinese news agency is one of only a handful of foreign news organisations allowed to operate in North Korea, which tightly controls the movement of all foreigners.
More details are emerging from accounts by Chinese villagers 20 kilometres (12 miles) away who said they were shaken by the blast.
"It was black smoke, just like the mushroom cloud after a nuclear explosion," one man said.
A local fisherman said: "I first saw a big fireball over the border... The sky in that area was full of yellow dust and dirt."
Pyongyang had requested international help before admitting publicly that an accident had happened.
North Korea "appreciates the willingness expressed by the governments of various countries and international bodies and organisations to render humanitarian assistance," said a statement released by the state news agency.
But the country's broadcast media have ignored the disaster so far.
An aid convoy travelled to the scene on Saturday with badly needed supplies including antibiotics, bandages and painkillers, said John Sparrow, a Red Cross spokesman in Beijing.
"We are fearful that they could be overwhelmed by the large numbers of injured," Mr Sparrow said.
He said North Korean Red Cross workers - until now the only aid officials allowed into the city - had spoken of massive destruction with nearly 2,000 homes destroyed and 6,000 badly damaged.
China and South Korea have each offered about $1m in emergency aid to their neighbour.
Several other countries, including the UK, have also offered aid.
The United States, which has described North Korea as part of the "axis of evil" with Iran and pre-war Iraq, said it was willing to help with the disaster relief if asked.
It is unclear what medical treatment is available to those injured in the disaster, as North Korea's medical system has all but collapsed and the country is experiencing severe shortages of food and energy.
The North Korean authorities have said the massive blast was the result of an "electrical contact caused by carelessness".
It appears two railway wagons carrying dynamite exploded after getting snagged on overhead electric cables.
WORLD'S WORST TRAIN DISASTERS
April 2004: Ryongchon, N Korea - Hundreds of casualties feared in station explosion
February 2004: Neyshabur, Iran - at least 300 killed when a runaway train explodes
June 2002: Dodoma region, Tanzania - at least 200 killed when passenger train collides with goods train
Feb 2002: Egypt - 300 killed in fire on train travelling to Cairo
June 1989: Ufa, Russia - More than 400 killed in gas explosion under two trains
Aug 1995: Uttar Pradesh, India - 300 killed in train collision
June 1981: Bihar, India - 800 killed when cyclone blows train into river
An Irish aid worker based in Pyongyang heard that the accident happened while the train was being shunted.
"They were trying to move these railway carriages to connect them to another train and they got caught in the overhead electric cable and that detonated the dynamite which caused the explosion," said Anne O'Mahony of Concern.
The accident happened just hours after the North Korean leader, Kim Jong-il, passed through Ryongchon on his way home from a trip to China.
He had been there to discuss North Korea's nuclear programme - a source of great tension with South Korea and that country's main ally, the US.