South Korea is demanding the return of a fishing boat and its crew, seized by North Korean authorities earlier this week.

South Korea's Unification Ministry said Wednesday the government sent a message to Pyongyang calling for the prompt release of the boat and the seven fishermen, based on international laws and customs, and on humanitarian grounds.

A spokeswoman for the ministry, Lee Jong-Joo, said the message was sent by South Korea's top Red Cross official to his counterpart in the communist North.

South Korean officials say the fishing boat, Daeseung 55, was captured Sunday off the peninsula's eastern coast.  It is not clear whether it had entered the North's exclusive economic zone.

Pyongyang has not commented on the incident.

Lee said the South Korean government has also asked for an explanation of why the fishing boat was seized.

Relations between the two Koreas have been tense since the sinking of a South Korean warship, the Cheonan, in March. South Korea responded by staging naval maneuvers with the United States off its east coast last month and was conducting another exercise off its west coast when the fishing boat was seized.

An international investigation concluded that a North Korean torpedo attack sank the Cheonan on March 26, killing 46 sailors. Pyongyang denies any connection with the sinking and says the report was fabricated.

Some information for this report was provided by AP and AFP.