North Korea today cut off key communications links with South Korea after vowing to sever all ties in protest at claims that it had sunk a Seoul warship. According to South Korea's unification ministry, the north had shut down communications lines between their Red Cross authorites and also lines connecting maritime officials of the two sides this morning. The Ministry, however , said North Korea has not so far cut military communication lines which are necessary for the operation of a Seoul-funded industrial site at Kaesong just north of the border. The North's state Committee for the Peaceful Reunification of Korea announced last evening that all ties are being severed.