Dandong, China ? North Korea on Saturday said that human “carelessness” contributed to the train blast that killed hundreds of people near the border with China, as international aid workers rushed to the site in response to the North’s appeal for help.

In its first statement on Thursday’s disaster, North Korea’s official news agency said the blast was touched off by “electrical contact caused by carelessness during the shunting of wagons loaded with ammonium nitrate fertilizer.”

The news agency for North Korea’s secretive government said that an investigation into the explosion at the railway station “so far shows that the damage is very serious.”

The statement also expressed appreciation for offers of international humanitarian assistance. Those offers came in the hours after the North issued a rare appeal for foreign help, inviting aid workers to come see the disaster site.

U.S. defense officials said damage from the blast extended at least 200 yards from the explosion at a railway station in Ryongchon, a city with chemical and metalworking plants and a reported population of 130,000.

American intelligence analysts thought it was probably a coincidence that the blast happened hours after North Korean leader Kim Jong Il reportedly passed through the station on his way home from a three-day visit to China.

There has been no unusual movement of North Korean military forces detected since the explosion, although it is likely some would aid in disaster recovery efforts, the officials said.

State Department spokesman Richard Boucher said the United States was evaluating the situation in North Korea to see “if there is a need or an opportunity for the United States to help.”

He noted that the United States had provided humanitarian aid in the past to North Korea. He added that there were no obstacles to sending assistance in response to the current situation.