Six US congressmen have begun a visit to North Korea, hoping to ease tensions in the crisis over the North's suspected nuclear weapons programmes.
Military tension on the border
They are the first US officials to be invited to the North since the nuclear crisis erupted in October, but they have made clear they are not travelling as envoys of the US Government.
The congressmen describe their visit as a "fact finding mission".
They have asked for a tour of the controversial nuclear complex at Yongbyon which has been at the centre of the nuclear stand-off, but it is not clear whether this will take place.
The North Korean authorities announced that they were reactivating the plant following a US decision to suspend oil shipments to the country over suspicions that a secret uranium enrichment programme was underway.
The congressmen are expected to meet the chairman of the North Korean parliament, Kim Yong Nam, and visit various institutions including a school, a factory and a church.
Territorial waters dispute
The delegation leader, Curt Weldon, said they would be making clear that the world was ready to help economically and provide humanitarian help, but only if the North Koreans were prepared to completely close down their nuclear programme.
The visit follows another warning from North Korea to the South over the alleged violation of territorial waters.
A statement on the official news agency said such provocative acts could lead to "irrevocable serious consequences".
The agency said four South Korean navy boats sailed into Northern waters on Thursday, following incursions by 16 warships on Wednesday and three on Tuesday.
South Korea has denied the accusations and said that fishing boats from the North had violated the sea border on three successive days this week.