Sources in China's border city of Dandong say a massive explosion occurred at a North Korean railroad station near the Chinese border that may have caused a large number of casualties.

The explosion reportedly happened about 1 p.m. Thursday in Ryongchon, a town 20 kilometers from China.

Details, including the exact number of casualties, are sketchy as the communist state cut off international telephone lines in an apparent attempt to prevent details from leaking out. But the Seoul-based Yonhap News Agency said some 3,000 people might have been killed or injured from the incident.

Yonhap said two fuel trains collided at the North Korean railroad station igniting a deafening explosion that rained debris for more than 16 kilometers around. As for the cause of the incident, reports in Seoul also said a gas storage near the railroad station had exploded.

The explosion is drawing much attention as it happened around nine hours after North Korean leader, Kim Jong-il, had passed by rail through the station as he returned from China. It was not clear whether the incident was related to Kim's journey.

Ryongchon, the site of the explosion, is a relatively densely populated strategic transportation center.