North Korea (searchsaid Saturday that careless handling of volatile ammonium nitrate (searchfertilizer contributed to the train blast that killed hundreds of people, 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 chemical is used in explosives and rocket fuel.

KCNA, the news agency for North Korea's secretive government, didn't elaborate further on the cause of the explosion at a railway station in Ryongchon (search), near the border with China.

It added that a preliminary investigation" shows that the damage is very serious."

U.S. defense officials said damage 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.

The news agency's 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.

On Saturday, an aid convoy was headed to the site carrying antibiotics, bandages, painkillers and other supplies — all of which are scarce in the impoverished country, said John Sparrow, a Red Cross spokesman in Beijing.

"We are fearful that they could be overwhelmed by the large numbers of injured," he said.

Sparrow said huge numbers of people might have been made homeless by the blast and would need tents and other shelter. North Korea has been wracked by famine in recent years, and "on top of the things we're already trying to do, we now have to deal with these new needs," he said.

United Nations agencies and European charities headed to the site. The group wasn't allowed to carry mobile communications, said Brendan McDonald, head of the U.N. office for coordination in Pyongyang, the North Korean capital.

The World Health Organization representative in North Korea, Eigel Sorensen, went to the site and the agency didn't expect to hear from him until he returned to Pyongyang at about 9 a.m. EDT Saturday.

"We have no information from him now," Dr. Jyoti Reddy said.

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 is 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 has provided humanitarian aid in the past to North Korea. He added that there are no obstacles to sending assistance in response to the current situation.

Many South Korean news organizations reported Saturday that hundreds of people may have died.

North Korean officials told Britain's ambassador that "several hundred people were thought to have died and several thousand were injured," a British Foreign Office spokesman said.

North Korean officials told diplomats and aid groups that more than 1,000 people were injured and thousands of apartments and houses destroyed or damaged in the blast.

"In Pyongyang, we already hold out our hands to the world community," North Korea's Deputy U.N. Ambassador Kim Chang Guk told Associated Press Television News. He said Pyongyang was seeking "generous help from the world community."

He said he didn't have further details about the explosion.

"I don't know what really happened, but I think it is very serious because our government held out hand to the world community for help," he said in New York. "It means it is a great incident."

He said Ambassador Pak Gil Yon officially requested U.N. help Friday. A U.N. mission, accompanied by several aid agencies, was to arrive at the disaster site Saturday to assess humanitarian needs and offer immediate support, U.N. spokesman Fred Eckhard said.

China's U.N. Ambassador Wang Guangya urged the international community to respond to the rare call for help from North Korea.

"This is unusual for DPRK. So I think that the international community should be forthcoming, be positive in responding to their request," he said.

"They got caught in the overhead electric wiring, the dynamite exploded, and that was the cause of the explosion," she said.

The blast leveled the train station, a school and apartments, South Korea's Yonhap news agency said, quoting Chinese witnesses. It said there were about 500 people in the station at the time.

South Korea's Chosun Ilbo newspaper, citing a South Korean intelligence source, said a U.S. spy satellite photograph showed damage mostly in densely populated neighborhoods east of the station, which included buildings for the military and ruling Workers' Party.

"Hospitals are jam-packed with people injured," Chosun Ilbo quoted a Chinese witness as saying.

There was no sign in Dandong, a Chinese border city about 12 miles from Ryongchon, of injured North Koreans. But the city's three biggest hospitals were preparing for a possible surge of patients.

"We're ready to offer our close neighbor our best medical help anytime," said an official at Dandong Chinese Hospital.

The explosion destroyed 1,850 apartments or houses and damaged 6,350, said Sparrow, citing officials in the North. The Red Cross reported at least 54 people killed and 1,249 injured, but Sparrow said "we are anticipating that the casualty figures will increase."

North Korea restricts the movement of foreigners, and groups that distribute aid to alleviate its food shortages are barred from some areas. Aid workers have been allowed to visit areas struck by drought or floods in recent years, but the government has never arranged such quick access to the scene of a disaster like the train explosion.