North Korea on Saturday blasted South Korea for hosting a multi-national naval drill aimed at preventing the transfer of weapons of mass destruction, calling it an "open declaration of war". South Korea for the first time played a full part in the Proliferation Security Initiative (PSI) on Thursday, with warships and aircraft from four countries staging an exercise off the southern port of Busan. The reclusive North, where leader Kim Jong-Il is paving the way for his son Kim Jong-Un to succeed him, is nuclear-armed.